wind tunnel study on fluctuating internal pressure of open

Wind and Structures, Vol. 32. No. 2 (2021) 105-114 DOI: https://doi.org/10.12989/was.2021.32.2.105 105

Copyright © 2021 Techno-Press, Ltd. http://www.techno-press.com/journals/was&subpage=7 ISSN: 1226-6116 (Print), 1598-6225 (Online)

1. Introduction

Previous investigations of wind disasters have shown

that windborne debris is one of the major sources of

building damage during severe storms (Minor 1994, Behr

and Minor 1994). The envelopes of buildings, such as doors

and windows, are vulnerable to the impact of windborne

debris. After the building envelope is perforated, an

originally sealed building turns into an open building, and

then the internal pressures of the building can increase

significantly and resonant response may also occur

(Shanmugasundaram et al. 2000, Lee and Wills 2002),

which may cause considerable damage to the building. Although a lot of research has extensively investigated

wind-induced internal pressures of buildings with openings,

most of it has generally focused on the internal pressures

induced by flow normal to the opening (Holmes 1980,

Stathopoulos et al. 1979, Kopp et al. 2008). However, the

fluctuating internal pressures induced by tangential flow

over openings from oblique wind directions are very

unsteady and may be more dangerous (Ai and Mak 2014).

Sharma and Richards (2003) stated that under tangential

Corresponding author, Ph.D. Professor E-mail: [email protected]

aPh.D. Student

E-mail: [email protected] bPh.D. Professor

E-mail: [email protected]

flow, excitation of the internal pressures at the resonant

frequency is affected by the “eddy dynamics” at the

opening. If the frequency of the “eddy dynamics” is matched to the Helmholtz resonance frequency, much

stronger resonance can be expected to take place. Guha et

al. (2011) investigated the internal pressures of opening

buildings by using a covariance integration approach, and

the results show that the internal pressure resonance at wind

angles of ±80 - 90° was driven by eddy at the opening due

to tangential flow excitation. Both of the above publications

suggest that the eddy of the incoming flow takes place at

the opening. However, if the eddy of the incoming flow did

emanate at the opening, the internal pressures of buildings

with a dominant opening would be affected similarly for all wind directions, which contradicts the phenomenon that the

internal pressure resonance is only excited at a particular

range of oblique angles, resulting in an increase in the

fluctuating internal pressures. In this paper, the term

“dominant opening” is understood to mean that the opening

in one face has an area at least twice the total area of the

openings in the other faces. Thus, the mechanism that

causes highly fluctuating internal pressures at some oblique

wind angles needs to be studied further.

In addition, the internal pressure of buildings with

openings is expected to be influenced by many other

factors, such as turbulence intensity (Iu), wind angle, opening location, background porosity and so on (Ginger et

al. 2008, 2013, Guha et al. 2013, Sabareesh et al. 2018).

There is still very little research being conducted to

determine the influence of these factors on the phenomenon

of the increase in fluctuating internal pressures observed

Wind tunnel study on fluctuating internal pressure of open building induced by tangential flow

Sheng Chen1a, Peng Huang1 and Richard G.J. Flay2b

1State Key Laboratory of Disaster Reduction in Civil Engineering. Tongji University. Shanghai 200092. China 2Department of Mechanical Engineering. The University of Auckland. Private Bag 92019. Auckland 1142. New Zealand

(Received October 3. 2020. Revised January 8. 2021. Accepted February 5. 2021)

Abstract. This paper describes a wind tunnel test on a 1:25 scale model of TTU building with several adjustable openings in

order to comprehensively study the characteristics of fluctuating internal pressures, especially the phenomenon of the increase in fluctuating internal pressures induced by tangential flow over building openings and the mechanism causing that. The effects of

several factors, such as wind angle, turbulence intensity, opening location, opening size, opening shape and background porosity on the fluctuating internal pressures at oblique wind angles are also described. It has been found that there is a large increase in

the fluctuating internal pressures at certain oblique wind angles (typically around 60° to 80°). These fluctuations are greater than those produced by the flow normal to the opening when the turbulence intensity is low. It is demonstrated that the internal

pressure resonances induced by the external pressure fluctuations emanating from flapping shear layers on the sidewall downstream of the windward corner are responsible for the increase in the fluctuating internal pressures. Furthermore, the test

results show that apart from the opening shape, all the other factors influence the fluctuating internal pressures and the internal pressure resonances at oblique wind angles to varying degrees.

Keywords: fluctuating internal pressure; opening building; wind tunnel test; oblique wind angle; external pressure fluctuations; internal pressure resonance

Sheng Chen, Peng Huang and Richard G.J. Flay

Fig. 1 Photograph of the rigid test model - TTU test

building, scale: 1:25

Fig. 2 Definition of wind directions

when there is tangential flow across the opening., Such

actions will increase the kinetic energy of the internal pressures, thus their ability to cause damage and this is

potentially a great threat to the safety of buildings.

Therefore, a detailed study of the fluctuating internal

pressure responses, including the internal pressure

resonances induced by tangential flow over building

openings is important to ensure the safety of buildings with

dominant openings in wind storms.

In this paper, the wind tunnel tests were carried out on a

rigid model of TTU building with several kinds of

openings. The phenomenon of the increase in the

fluctuating internal pressures under tangential flow and the mechanism causing that were investigated in detail. In

addition, the influencing factors on the fluctuating internal

pressures and their resonant responses, such as turbulence

intensity, opening location, opening size, opening shape and

background porosity of the building, were systematically

determined for a range of wind directions.

2. Wind tunnel tests

2.1 Test model procedure The wind tunnel tests were conducted in the TJ-2

atmospheric boundary layer wind tunnel at Tongji

University. It is a closed return-flow wind tunnel with a

rectangular test section. The Texas Tech University (TTU)

building of WERFL (Texas Tech Wind Engineering

Research Field Laboratory) is a typical low-rise building

and many previous researchers have used it as the prototype

to study the wind-induced internal pressures of buildings

with openings (Levitan and Mehta 1992, Ginger 2000,

Guha et al. 2013). Thus, the TTU WERFL building was

also adopted as the full-scale prototype for the wind tunnel rigid model discussed in this paper so that the present

findings could be compared with previously published

findings. The geometric scale of the model is 1:25 and the

Table 1 Model configurations tested in the wind tunnel

Designation A31 A53 A68 A81 A37 A74 As

Opening width

× height (mm×mm)

31×31 53×53 68×68 81×81 37×74 74×37 53×53

Opening fraction

1% 3% 5% 7% 3% 3% 3%

Opening aspect ratio

1:1 1:1 1:1 1:1 1:2 2:1 1:1

Opening location

center center center center center center side

Fig. 3 Layout of pressure taps and background porosity

external dimensions of the model are 548 mm long, 364

mm wide and 160 mm high. This TTU test model was made

of two-layer Plexiglas plates, and the pressure tap tubing

was sandwiched between the two plates to reduce its

interference on the model internal pressures. The two-layer

plate thickness is 10 mm, so the internal dimensions of the

model are 528 mm long, 344 mm wide and 150 mm high

and the internal volume of the test model is 0.027 m3. It should be noted that the velocity scale of the test and the

prototype is 1:1 in this study, so the internal volume of the

model doesn’t need to be scaled (Holmes 1979).

The dominant opening was located on one of the smaller

walls 364 mm wide × 160 mm high, and this wall could be

replaced by others with different opening geometries, as

shown in Fig. 1. The 0° wind direction in the wind tunnel

tests is defined as flow normal to the wall with the opening,

as shown in Fig. 2. The model was tested at wind directions

varying from 0° to 180°, at intervals of 5° from 0° to 90°

and intervals of 10° from 90° to 180°. There were a total of 90 or 92 channels of pressure taps

in the test models, including 20 internal taps on the roof, 15

internal taps on each of the left and right walls, 12 internal

taps on the leeward wall, and 20 or 22 internal taps and 8

external taps on the wall with the opening. It should be

noted that the 8 pressure taps around the opening were

“two-sided” pressure taps which could measure both

internal and external pressures. The layout of pressure taps

is shown in Fig. 3. Table 1 summarizes the different

designations of the configurations tested in the wind tunnel.

The special openings including A37, A74 and As are shown in

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(a) A37

(b) A74

(c) As

Fig. 4 Special opening shapes and locations

Fig. 4, and the opening fraction is defined as the ratio of the

size of the opening to the 2/3 power of the internal volume

of the model, which results from dimensional analysis. Note

that for opening As, the model was tested not only from 0°

to 180° but also from 0° to -180°. Note that all of the orifice

shapes discussed in this paper are rectangular because most

of the vulnerable openings of buildings, such as doors and

windows, are rectangular.

In addition, for a typical nominally sealed building, the background porosity (defined as the ratio of effective

leakage area to the surface area of the building) ranges from

10-4 to 10-3 (Ginger et al. 1997). The total surface area of

the test model is 491,312 mm2, so the area of the

background porosity should be ~ 49 - 490 mm2. To simulate

the background porosity of the model, 10 × 8 mm diameter

holes were drilled uniformly on the leeward wall, as shown

in Fig. 3. The area of each hole is about 50 mm2, so the total

area of the background porosity of the test model is 500

mm2. During the wind tunnel tests, a range of background

porosities, 0%, 0.01%, 0.05% and 0.10% were simulated by sealing appropriate numbers of holes.

2.2 Simulation of uniform wind fields

The wind fields simulated in the wind tunnel tests were

all uniform to make sure the turbulence intensities in the

vicinity of the opening were spatially uniform. In order to

simulate uniform turbulent wind fields, two 200 mm wide

vertical barriers were positioned upstream of the test model

to generate vortices in the flow. Different turbulence

intensities in the wind fields could be simulated by adjusting the distance between the two barriers and the

distance between them and the windward wall of the test

model. However, the turbulence intensities near the floor of

the wind tunnel were quite different from those at greater

heights due to the influence of the floor boundary layer. To

eliminate that influence and to make sure that the

turbulence intensities in the vicinity of the opening were

uniform, a circular test platform with a 1000 mm diameter

(as shown in Fig. 5) was set up to raise the bottom of the

test model 200 mm above the wind tunnel floor.

(a) Type 2 (b) Type 3

Fig. 6 Photographs of the model in the wind tunnel showing

the upstream barriers used to generate the uniform turbulent onset flow for Type 2 and Type 3

Fig. 5 Photograph of 1000 mm diameter test platform 200

mm above the wind tunnel floor

In total there were three different uniform wind fields

simulated in the wind tunnel tests. Type 1 wind field was an empty wind tunnel without any barriers; Type 2 and Type 3

wind fields were simulated using the two 200 mm wide

barriers with different spacings. As depicted in Fig. 6, for

the Types 2 and 3 wind fields the distances between the two

barriers were 1.0 m and 0.8 m, and between the barriers and

the windward wall of the model were 9.0 m and 3.5 m,

respectively. The wind speed at the open wall of the test

model was always set to 10 m/s in all three wind fields.

Vertical profiles of mean wind speed and turbulence

intensity, and the longitudinal wind speed spectra measured

at the open wall location at a height of 80 mm without the

model in position are shown in Fig. 7. It can be seen that the mean wind speed and the turbulence intensity are smaller

near the platform floor, but both of them are generally

constant over the ~ 40 - 120 mm maximum vertical height

range of the openings, except that the turbulence intensity

profile for the Type 3 wind field shows small degree of non-

uniformity in the 40 - 80 mm height range. At the openings,

the turbulence intensities for the Types 1, 2 and 3 wind

fields are about 0.014, 0.094 and 0.176, respectively. The

longitudinal wind speed spectra have been fitted by the von

Karman spectral shape. Note that the integral length scale

measured in the wind tunnel was three to five times smaller, but was at the same order, compared with that in full scale

derived from ESDU (1985) and Flay and Stevenson (1988),

which is a common issue due to the constraints of the wind

tunnel cross section (Pan et al. 2013, Liu et al. 2019).

Hence this study is primarily focused on bluff body

aerodynamics and understanding how the internal pressure

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coefficients change with the variables described in Section

1, and it is not a case study performed on the TTU WERFL

building subjected to a corresponding simulation of the

atmospheric boundary layer flow.

2.3 Sampling and processing the pressure data

The model pressure taps in these tests were sampled simultaneously at 312.5 Hz for 28.8 s, resulting in a total of

9000 samples from each pressure tap. The wind pressure

time histories, ( )P t , obtained from wind tunnel tests were

converted to dimensionless wind pressure coefficients

( )PC t using Eq. (1)

where ( )PC t is the pressure coefficient and ( )P t is the

pressure at the pressure tap. ( )staP t and ( )tolP t are the

reference static and total pressures at the ridge height of the

building, respectively. It is known that the wind pressure is a stationary random

variable, so the mean and standard deviation of the pressure

coefficients of the taps were calculated using Eqs. (2)-(3),

respectively

,1

1 N

P P kkC C

N (2)

(3)

where N=9000, and PC and 𝐶�̃� are the mean and

standard deviation wind pressure coefficients, respectively.

(a) Type 1

(b) Type 2

(c) Type 3

Fig. 7 Vertical profiles of mean wind speed and turbulence intensity, and longitudinal wind speed spectra at a height of 80 mm for the three onset wind flows

( ) ( )( )

( ) ( )

staP

tol sta

P t P tC t

P t P t

(1)

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Fig. 9 Internal pressure coefficient spectra at wind angles of

60° and 70° for Iu=0.014

3. Discussion of wind tunnel test results

3.1 Increase in fluctuating internal pressures

The external and the internal pressures in this paper are

derived from the average values of all the relevant external

and internal pressure taps. The variations of fluctuating

internal pressure coefficients of the test model as a function

of wind direction in the three uniform wind fields with

turbulence intensities of 0.014, 0.094 and 0.176 are depicted in Fig. 8. The size of the opening in the model used for

these results is 53 × 53 mm (A53) and the test wind speed is

10 m/s. It should be noted that in this figure and

subsequently, the term “fluctuating internal pressure

coefficient” refers to the “standard deviation of the pressure

coefficient time history”.

It can be seen that the turbulence in the incoming flow is

one of the main excitation sources for the fluctuating

internal pressures in this model as higher turbulence leads

to higher internal pressure fluctuations and vice versa.

Evidently, there is an increase in the fluctuating internal

Fig. 10 Internal and external pressure coefficient spectra at a wind angle of 70° for Iu=0.014

pressure coefficients at oblique wind angles from about 60°

to 90° in all three wind fields. When the turbulence

intensity is relatively high (0.094 and 0.176), the peak value

of the fluctuating internal pressure occurs when the flow is

normal to the face, i.e., at 0°. However, at the lower

turbulence intensity of 0.014, the peak value of the

fluctuating internal pressures occurs at 80°, and is therefore

excited by flow that is essentially tangential to the opening,

resulting in a fluctuating internal pressure coefficient that is 2.5 times higher than that from flow normal to the face with

the opening. Thus, it can be concluded that under tangential

flow, there is an increase in the fluctuating internal

pressures, and the increased fluctuations at oblique angles

may be much higher than those from angles normal to the

opening when the turbulence intensity is low.

The results shown in Fig. 8 suggest that compared to the

fluctuating internal pressure resulting from flow normal to

the face with the opening, the most significant increase in

the fluctuating internal pressures takes place in the wind

field with a low turbulence intensity of 0.014 at oblique wind angles from 60° to 80°. Therefore, the results

discussed below are all for the lowest turbulence intensity

Type 1 wind field with Iu=0.014.

The internal pressure coefficient spectra for the wind

directions of 60° and 70° are compared in Fig. 9 to try to

determine the mechanism causing the increase in the

fluctuating internal pressure coefficients at oblique wind

angles. It shows that the energy in the frequency region of

0~100 Hz for the 70° wind direction data is significantly

greater than that for the 60° direction. Also, there are two

obvious resonant peaks in the 70° results (at frequencies of about 30 Hz and 65 Hz), while there is only one smaller

resonant peak at 60° (at about 65 Hz). According to

previous research, it is generally accepted that a building

with a dominant opening can be treated as a Helmholtz

acoustic resonator (Holmes 1979, Vickery and Bloxham

1992), so the resonant peak at 60° and one of the resonant

peaks at 70° may be generated by internal pressure

Helmholtz effect. The Helmholtz resonance frequency can

be calculated from Eq. (4) (Liu and Saathoff 1981)

0

0

1

2

aH

a e

cA Pf

L V

(4)

Fig. 8 Fluctuating internal pressure coefficients as a function of wind direction for the three turbulent wind

fields

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Fig. 11 Internal pressure coefficient spectra in three

different turbulence intensities at 70°

Fig. 12 Fluctuating internal pressure coefficients of the

model with different opening locations as a function of

wind direction

in which =1.4 is the ratio of specific heats for air; 0A is

the opening area; aP is the ambient pressure;

a is the air

density; 0 0e IL L C A is the effective length of the air

slug at the opening, where 0L is the length of the opening

and IC =0.886;

0V is the internal volume of the building;

c =0.6~1.0 is the discharge coefficient for the opening.

Based on Eq. (4), the Helmholtz frequency of the test

model with the A53 opening is 56.35 - 72.75 Hz, which

indicates that the peak at the higher frequency (about 65

Hz) in the 70° data is generated by Helmholtz resonance,

and the resonance at the lower frequency (about 30 Hz) is excited by something else, perhaps “eddy dynamics” of the

external flow (Sharma and Richards 2003). It is worth

noting that the peak in the spectrum for the wind direction

of 60° is also generated by Helmholtz resonance, but it is

much smaller than that for the wind direction of 70°.

In order to determine the cause of the resonance at the

lower frequency of 30 Hz in the 70° data, internal pressure

coefficient spectrum and external pressure coefficient

spectrum from pressure taps near the opening are plotted in

Fig. 13 Internal pressure coefficient spectra for different

opening locations for a wind direction of 70°

Fig. 14 Internal pressure coefficient spectra for different

opening locations for a wind direction of 80°

Fig. 10. It shows that there is also a resonant peak at about

30 Hz in the external pressure coefficient spectrum. This

indicates that the resonance at about 30 Hz in the internal pressure coefficient spectrum at 70° is excited by the

external flow fluctuations, which have been transmitted

through the orifice by the pulsating flow. However, the

source of the external pressure fluctuations needs to be

studied further. They may emanate from eddies at the

opening, or from flapping shear layers on the side of the

building downstream of the windward corner.

Furthermore, in order to evaluate the influence of

turbulence intensity on these resonant responses of the

internal pressures, Fig. 11 compares internal pressure

coefficient spectra from the three different wind fields at the wind angle of 70°. It can be seen that in the low frequency

region (<25 Hz), the spectra in different turbulent wind

fields vary greatly, and the fluctuating energy of the internal

pressures increases with increase in the turbulence intensity.

However, in the high frequency region (>25 Hz), the spectra

show little difference, especially at the Helmholtz resonant

peak at about 50 – 60 Hz. In addition, the resonant peak at

about 30 Hz, which is induced by the external pressure

fluctuations is also nearly unchanged by turbulence

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Fig. 15 Fluctuating internal pressure coefficients with

different opening sizes as a function of wind direction

intensity. Therefore, it can be concluded that the turbulence

intensity of the approach flow only affects the internal

pressure coefficient spectra at low frequencies less than 25 Hz, but has no influence on the internal pressure resonances

induced by external pressure fluctuations and Helmholtz

effect. This also indicates that the external pressure

fluctuations causing the internal pressure resonance at about

30 Hz are not from turbulence in the incoming flow.

3.2 Source of external pressure fluctuations

Earlier research (Sharma and Richards 2003, Guha et al.

2011) has suggested that “eddy dynamics” of the incoming

flow emanating at the opening are responsible for the internal pressure resonance. If the eddy of the incoming

flow does emanate from the opening, then it could be

argued that the internal pressures of buildings with a

dominant opening would be affected by such “eddy

dynamics” similarly for all wind directions and opening

positions. However, the present results appear to show that

pressure fluctuations from the external flow field only have

a large influence on the internal pressures of the building

through a particular range of oblique wind angles. This

suggestst that the mechanism for this excitation is different

from “eddy dynamics” at the opening, which could be

happening at the same time. Since the strength of the pressure fluctuations in the external separated flow from an

upstream corner along a sidewall depends on wind

direction, it seems plausible that such highly fluctuating

unsteady flow could influence the internal pressures of a

building with a dominant sidewall opening, depending on

the wind direction.

In order to verify this hypothesis, fluctuating internal

pressure coefficients were measured using models with

different opening locations and are compared in Fig. 12.

The “center” opening is A53 (see Table 1) for directions 0°

to 180°, and the “close to and far away from upstream wall corner” opening is As for directions 0° to -180° and 0° to

180°, respectively. Fig. 12 shows that the wind directions

producing an increase in the fluctuating internal pressures

depend on the opening locations. As the wind direction

increases from 0° (normal to the opening), the opening

Fig. 16 Internal pressure coefficient spectra with different

sized openings for a wind direction of 70°

closest to the upstream wall corner first produces an

increase in the fluctuating internal pressures at 65°.

It is hypothesized that the reason for this is that at 65°,

the flow separates at the windward wall corner, and the

region of wall close the upstream corner is periodically in

the separation zone, with its associated highly fluctuating pressures as the separation zone grows and contracts. The

region of the wall further away from the upstream corner

remains in the area of flow reattachment with its associated

much steadier pressures. When the opening is located in the

area of intermittent flow separation, the fluctuating

pressures can be transmitted into the cavity through the

opening and lead to enhanced fluctuations in the internal

pressure at the low excitation frequency. However, if the

opening is located downstream of reattachment, the steadier

pressures there would not be expected to cause enhanced

pressure fluctuations in the cavity.

When the angle is increased to 80°, the whole sidewall containing the opening can be expected to be in highly

fluctuating separated flow, and so even the opening far from

the windward corner will be subjected to highly fluctuating

pressures. These fluctuating pressures are transmitted to the

cavity through the opening and are thus expected to cause

an increase in the fluctuating internal pressure coefficients,

as was observed in the experiments, and is shown in Fig.

12. Therefore, the further the opening is from the windward

corner, the larger the expected wind angle has to be to cause

an increase in the fluctuating internal pressure coefficients.

To further verify that the source of the external pressure fluctuations is separation of the flow from the windward

corner, Figs. 13-14 compare the internal pressure spectra of

the model with different opening locations for wind

directions of 70° and 80°, respectively. Fig. 13 shows that

for the wind direction of 70°, the closer the opening is to the

upstream corner, the larger the internal pressure fluctuations

and the stronger the low frequency resonance (0 - 30 Hz)

and Helmholtz resonance (60 - 70 Hz) in the pressure

spectrum. The opening closest to the upstream corner is

subjected to stronger low frequency external fluctuations

which are transmitted into the cavity, which also strengthen the Helmholtz resonance. Noticeably, both of the resonant

111


Fig. 17 Fluctuating internal pressure coefficients with three

rectangular openings with the same area but different aspect

ratios, as a function of wind direction

frequencies in the internal pressure spectra do no change

with changes in the opening location. In Fig. 14, it can be seen that for the wind direction of 80°, there are two strong

resonance peaks for all three opening locations, so the

openings at all three locations are in similar strength

separated flow for this wind direction, and thus there are no

changes in the internal pressure fluctuations associated with

Helmholtz resonance and the separated shear layer on the

sidewall.

In summary, it appears that for certain oblique wind

directions, the external pressure fluctuations in the

separated flow on a sidewall downstream of the windward

corner can be transmitted through an opening and enhance the internal pressure fluctuations, including the Helmholtz

resonance. This enhancement depends on the wind direction

and the location of the opening on the sidewall. The closer

the opening is to the upstream corner, the smaller the wind

angle causing an increase in the fluctuating internal

pressures.

3.3 Effects of other factors

3.3.1 Opening size The opening size is one of the most important factors

influencing the internal pressures of buildings with an opening. In Fig. 15, the fluctuating internal pressure

coefficients with different opening sizes (or opening

fractions) are plotted as a function of wind direction. It

illustrates that the fluctuating internal pressure coefficient

increases with increase in opening size not only in normal

flow (0°), but also at the oblique wind angles, including

those causing an increase in the fluctuating internal

pressures (60° to 90°). The reason for this is that an increase

in opening size reduces the damping at the orifice acting on

the pulsating flow there, and thus the responses of the

internal pressures to external excitation are strengthened (Ginger et al. 2010).

To investigate the influence of opening size on the

internal pressure resonances induced by external pressure

fluctuations and Helmholtz effect at oblique wind angles,

Fig. 18 Internal pressure coefficient spectra with different

opening shapes at 80°

Fig. 19 Fluctuating internal pressure coefficients of the

model with different background porosities as a function of

wind direction

the internal pressure coefficient spectra for different sized

openings at 70° are compared in Fig. 16. It shows that both

of the resonant responses of the internal pressures are

amplified by the growing opening sizes and the Helmholtz

frequency is also increased, as expected according to Eq. (4). The results indicate that the size of the opening plays an

important role in the fluctuating responses of the internal

pressures. With larger opening sizes, there is less damping

at the entrance, resulting in a greater transmission of the

external pressure fluctuations into the cavity, and also an

increase in both the Helmholtz resonance magnitude and its

natural frequency.

3.3.2 Opening shape Fig. 17 depicts fluctuating internal pressure coefficients

with three different aspect ratio rectangular openings with the same area. The results show that there is negligible

effect of shape when the openings are the same size. As

shown in Fig. 17, similar to earlier results, there is also a

large increase in the fluctuating internal pressure

coefficients as the wind direction changes from 60° to 80°.

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Fig. 20 Internal pressure coefficient spectra with and

without background porosity at 75°

Thus, the influence of the opening shape on the

fluctuating internal pressures of buildings with openings

can be ignored.

To confirm the expectation that the effect of opening

shape on the internal pressure coefficient spectrum would be small or negligible, they were determined and are

illustrated in Fig. 18. It shows that all the spectra are very

similar. Therefore, rectangular openings with the same size

but different aspect ratios have little influence on the

fluctuating internal pressure spectrum across the whole

frequency range of interest, including the resonances


effect.

3.3.3 Background porosity Fig. 19 plots fluctuating internal pressure coefficients of

the model with opening A53, with and without various

background porosities, in order to determine the effect of

background porosity on the fluctuating internal pressure

response at different wind angles. It can be seen that an

increase in background porosity produces a reduction in the

magnitude of the fluctuating internal pressure coefficients

across all wind directions. This effect is especially evident

for the wind direction of 75°, which produces the highest

value, where the fluctuating internal pressure without

background porosity is much higher than that with

background porosity. These results demonstrate that, as

expected, an increase in background porosity causes an increase in the damping of the internal pressure system (Yu

et al. 2012), thereby reducing the fluctuating internal

pressure coefficients, especially at the oblique angle

producing the highest value. This suggests that background

porosity could have a significant effect on the internal

pressure resonances at oblique wind angles. Thus, to

investigate this question, spectra were calculated and Fig.

20 compares internal pressure coefficient spectra with and

without background porosity for the wind direction of 75°.

It is obvious in Fig. 20 that the spectral density is

weakened slightly across the frequency range 0 to 90 Hz by the background porosity. However, the resonant frequencies

in the internal pressure spectra are unchanged. Therefore, it

can be concluded that the damping effect of background

porosity reduces the fluctuating internal pressures of the

buildings with an opening, including both of the internal

pressure resonant responses induced by external pressure

fluctuations and Helmholtz effect, but that their resonant

frequencies are unchanged.

4. Conclusions

This paper discusses wind tunnel measurements aimed

at investigating the phenomenon of the increase in the

fluctuating internal pressures of buildings with a dominant

opening under tangential flow compared to normal flow,

and the mechanism causing that. A series of factors

including turbulence intensity, opening location, opening

size, opening shape and background porosity have also been

analyzed to determine their effects on the fluctuating

internal pressures subjected to wind flow from 0° to 180° and the internal pressure resonances at oblique angles. The

major conclusions from this study can be summarized as

follows:

• There is an increase in the fluctuating internal

pressures of buildings with a dominant opening through

a particular range of oblique wind angles. This is

attributed to the response of the internal pressure to the

increase in the external pressure fluctuations at wind

angles when the flow separates from the sidewall, and

the orifice lies under the strongly fluctuating shear layer.

The closer the orifice is to the upstream corner, the

smaller the angle causing an increase in the fluctuating internal pressures. In addition, the highly increased

fluctuating internal pressures at oblique flow directions

can be greater than those excited by relatively low

turbulence intensity flow normal to the wall with the

opening.

• The fluctuating internal pressures of buildings with a

dominant opening increase with increase in the approach

flow turbulence intensity, but the turbulence intensity of

the flow has no influence on the internal pressure

resonances induced by building-induced external

pressure fluctuations and Helmholtz effect at the oblique wind angles. The wind directions which cause an

increase in the fluctuating internal pressure are different

for different opening locations, but both of the

frequencies of the internal pressure resonances at the

oblique angles also do not change with changes in the

opening location.

• The aspect ratios of rectangular openings have

virtually no influence on the fluctuating internal

pressures of buildings with an opening for all wind

directions, nor on the internal pressure resonances


effect at oblique angles. However, the effects of opening size and background porosity on the fluctuating internal

pressures of buildings with an opening are very evident

due to their influence on the damping of the internal

pressure system. The fluctuating responses of the

internal pressures, including the magnitude of the

resonances induced by the external pressure fluctuations

and Helmholtz effect, are amplified by increasing the

opening size but are reduced by increasing the

113


background porosity. In addition, the Helmholtz

resonance frequency also increases with larger opening

sizes, as expected, but remains unchanged with changes

in background porosity.

Acknowledgments

The research described in this paper was financially

supported by the Chinese National Natural Science

Foundation (51678452) and the Ministry of Science and

Technology of China (Grant No. SLDRCE19-B-12).

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