By

ANIRUDH B

11MN01

Subsonic wind tunnel design methodology

What is a wind tunnel?

Wind tunnels are measurement tools to study gas flows around a body and the forces generated by the gas-body interaction

Mostly air is used in wind tunnels

Using such tool it is possible to measure global and local flow velocities, as well as pressure and temperature around the body

Components of a wind tunnel

Bell mouth section/Nozzle Fine screen mesh

Components of a wind tunnel

Test chamber Diffuser section with a Suction fan located at The left of the diffuser

Wind tunnel Design

Closed circuit wind tunnel

Wind tunnel Design

Test Chamber: Dimensions, shape and air velocity

Wind tunnel Design

Wind tunnel dimensions are directly related to the test chamber cross section

In this case, a square cross section of 0.5m and air velocity of 30m/s is used.

Hydraulic diameter, is calculated.Test chamber length should be 0.5 to 3 times the

hydraulic diameter.In this case, the test chamber length is taken as 2 times Chamfer the sharp edges to 45 deg in order to avoid the

air velocity reduction and increase in boundary layer thickness.

Test Chamber: Dimensions, shape and air velocity

Wind tunnel Design

Nozzle: Area ratio

Wind tunnel Design

Nozzle area ratio should be between 6-10 to avoid pressure losses through the screens mounted between the settling chamber and the nozzle

In this case, area ratio of 7 is chosen.Nozzle’s silhouette is defined by Bell-Mehta’s polynomial

equation

Where, and y=h and

L is the total axial nozzle length and h is half the cross-section side-length

Nozzle: Area ratio

Wind tunnel Design

To determine the Bell-Metha polynomials, the following boundary conditions are imposed.

Nozzle: Area ratio

Wind tunnel Design

Nozzle: Area ratio

Experimentally it was evident that causes the air flow to detach at the nozzle exit and increases the boundary layer thickness.

Therefore, the ratio was set to 0.91 and the nozzle length of 1.3m was obtained.

Wind tunnel Design

Second Diffuser

Wind tunnel Design

Inlet cross sectional area is governed by the fan dimensions

Ratio between fan cross sectional area and test section cross section area should lie between 2 to 3.

If the ratio goes beyond 3 then irregular flow velocities may be generated and if it goes lesser than 2 the wind tunnel dimensions become larger.

In this case the ratio is chosen as 2.

Second Diffuser: Inlet cross sectional area

Wind tunnel Design

Air velocity at the fan outlet can be calculated by

Air flow at the fan outlet yields 15m/s.From the area ratios, we can find the fan cross section

diameter and in this case it is found to be 0.8mNow the inlet and outlet cross section of the diffuser is

known.

Second Diffuser: Inlet cross sectional area

Wind tunnel Design

Equivalent cone expansion angle can be determined by

Where, is the inlet section’s hydraulic diameterAs per the design rule of subsonic diffusers the total

included angle for constant area ratio, length should not exceed 6 deg.

Therefore to keep the length of the diffuser to be minimum, 3 deg was chosen and the above eq. can be solved for minimum length of the diffuser (6.58m in this case)

Second Diffuser: Inlet cross sectional area

Wind tunnel Design

Shape adapter is also designed and its total Length equals 0.3m

Shape adapter: Length

Wind tunnel Design

side l of the outlet CS area can be calculated by

l is found to be 0.71m for this case study with the max. cone angle of 4 deg the length of the frst diffuser can be found by the same eq. used for the second diffuser.

length of the first is found to be 3.32m

First Diffuser: side l of the outlet CS area

Wind tunnel Design

First Diffuser: side l of the outlet CS area

Wind tunnel Design

side l of the outlet CS area can be calculated by

l is found to be 0.71m for this case study with the max. cone angle of 4 deg the length of the frst diffuser can be found by the same eq. used for the second diffuser.

length of the first is found to be 3.32m

First Diffuser: side l of the outlet CS area

Wind tunnel Design

Corners are equipped with blades or bent flat plates to minimise the turbulence in four corners

Chord value is determined by

Where, 0.25 is the vane gap-chord ratio and is the vane gap and given by the equation,

Where 25 is the no. of bent flat plates ( acc. To design rule)

Corners: small corners and large corners

Wind tunnel Design

Minimum bent flat plate radius is given by,

Where, is the central angle

subtended by the chord

Corners: small corners and large corners

Wind tunnel Design

Settling chamber consists of honeycombs and mesh screen to reduce the flow turbulence before it enters the nozzle.

Settling chamber

Wind tunnel Design

Settling chamber: honeycomb

Key design factors are:1) Length2) Hydraulic diameter3) porosity

Wind tunnel Design

Settling chamber: honeycomb

Porosity is given by,

is the flow CS area and is the total CS area

Criteria's to be verified in honeycomb design is,

Where is given by,

Wind tunnel Design

Settling chamber: honeycomb

solidity is given by,

is the sheet CS area and is the total CS area

Wind tunnel DesignSettling chamber: honeycomb

Where,

Metal sheet divisions,

No. of divisions height wise,

No. of divisions width wise,

Wind tunnel DesignSettling chamber: screens

To have effective reducing in turbulence the porosity must lie between 0.58 to 0.8

Different mesh qualities (coarse, medium, f ine)are efficient than single fine mesh

Wind tunnel DesignPressure losses

1) Pressure loss in constant cross section area sections (friction)2) Pressure loss in diffusers (friction and expansion)3) Pressure loss in corners (friction and expansion)4)Pressure loss in screens(porosity or its complement solidity, the Reynolds number Calculated with wire diameter, and mesh factor)5) Pressure loss in honeycombs (length to cell hydraulic diameter ratio, porosity and Reynolds number)6) Pressure loss in nozzles (skin friction)

Wind tunnel Design

Fan selection

SummaryThe design procedure consists of the following main steps:

1. Defining the test section dimensions and desired flow velocity by test type;2. Wind tunnel component design by test section criteria;3. Wind tunnel component pressure loss calculation;4. Determining pressure loss throughout the wind tunnel circuit as a function of the possible flow velocity in the testing section in both open and closed configurations;5. Matching wind tunnel components to commercial fans, and energy considerations.

References:

1)Justin D Periera, “Wind tunnels-Aerodynamics, models and experiments”, Nova science publishers2) Metha R. D., Bradshaw P. “Design Rules for Small Low Speed Wind Tunnels” Journal of Royal Aeronautical Society 1979, Vol. 73.3)Wind tunnel design and wind tunnel parts-NASA