Table of Contents

1.0  Introduction						2

2.0 Approach 3

3.0 Experimental Configuration 4

3.1 Nozzle and Straight Section Design 4

3.2 Diffuser Design 5

3.3 Bump Design 5

3.4 Actuator Design 5

3.5 Velocity and Frequency Design 6

4.0 Test Matrix 6

5.0 Equipment and Materials 7

6.0 Data Collection and Analysis 7

7.0 Procedure and Schedule 8

8.0 References 9

List of Figures

1. Diffuser Configuration 2

2. Effect of Bumps 2

3. Flow Regimes in Two-Dimensional Straight Walled Diffuser 3

4. Schematic of Components and Stand 4

5. Dimensions of Components 4

6. Speaker Locations 6

7. Test Matrix 6

8. Work Schedule for Spring Term and Fall Terms 8


A diffuser is used to decelerate the air flow entering an engine. The performance of a diffuser is measured by the pressure gradient across the diffuser and is determined by the divergence angle. The maximum divergence angle of a diffuser is limited by flow separation. In this project, a "bump" will be placed in the diverging wall of the diffuser where the separation of the flow is expected to occur in the attempt to delay separation. A disturbance created by the actuator, a loudspeaker, will impinge on the "bump" and create a scaled-down disturbance closer in size to that of the separating flow section along the wall of the diffuser. The experiment will determine if this new, smaller disturbance is effective in delaying the flow separation and thus improving diffuser performance.

1.0 Introduction

Diffusers are used in engines to decelerate the entering air flow. The amount of deceleration and hence the performance of a diffuser is primarily dictated by the divergence angle, shown in Figure 1. For a fixed inlet width, a larger divergence angle will result in better performance, usually defined by a higher pressure gradient along the length of a diffuser. The divergence angle is limited by the advent of stall, flow separation along the walls of the diffuser. The separated region acts as a blockage and narrows the effective width of the diffuser. As a result, stall leads to decreasing pressure recovery and performance.

	Figure 1:  Diffuser Configuration

Previous research has shown improvements in diffuser performance using air jets to impart momentum to the flow and flaps and speakers to introduce oscillations in the flow[1-4]. The oscillations introduced in the flow by a speaker may be improved by placing "bumps" in the diverging wall of the diffuser. Research has shown that bumps increase the acoustic receptivity of the boundary layer[5]. The bumps could make the boundary layer of the diffuser more responsive to oscillations. A bump is approximately 100 microns high and 1 cm wide.

	Figure 2:  Effect of Bumps

The objective of this project is to determine the effectiveness of the bumps in delaying flow separation. The bumps will be placed where the flow is expected to begin separating. According to research, this is the area where greatest receptivity is predicted to occur.[5] The oscillations produced by the loudspeaker, as shown in Figure 2, will impinge on the bump and generate scattered, scaled-down oscillations closer in size to that of the separating flow section along the wall of the diffuser. This scaled-down oscillations will make the separated region oscillate. Once these oscillations reach resonance, the high amplitude of the oscillations could make the flow reattach. The delay of flow separation may enhance diffuser performance but may also provide structural and economic advantages by allowing larger divergence angles. For a given flow velocity, a larger divergence angle would result in a shorter, lighter, and hence, cheaper diffuser.

Open-loop tests will be conducted in the 1' X 1' wind tunnel at MIT. The efficiency of the diffuser with the bumps will be tested in the region of transitory stall. The range of diffuser divergence angles is dictated by the ratio N/W1 of the diffuser as shown in Figure 3.

	Figure 3:  Flow Regimes in Two-Dimensional
	              Straight Walled Diffuser[3]

2.0 Approach

Since the Written Proposal, there have been several changes to the project approach. Instead of conducting open-loop and closed-loop tests, we will only be performing open-loop testing due to the time constraint. Instead of developing an actively controlled diffuser, the focus will be on determining the effectiveness of the bumps in delaying stall. We will vary the number of these bumps and the spacing between them for three different divergence angles of 5, 10, and 15 degrees. In the experimental setup, we have added a straight section between the nozzle and the diffuser to make the flow transition smooth. Also, the speaker will be placed in two different positions: mounted on the top wall of the diffuser and at a distance from the diffuser exit.

3.0 Experimental Configuration

The experimental configuration is composed of a nozzle, straight section, diffuser and stand. It was designed to be used with the 1 foot by 1 foot wind tunnel. A schematic of this setup is shown in Figure 4.

	Figure 4:  Schematic of Components and Stand

3.1 Nozzle and Straight Section Design

The purpose of using a nozzle instead of attaching a diffuser directly to the wind tunnel is to obtain a smooth flow in the diffuser inlet. Also, the exit of the nozzle will require a smaller diffuser than the plain exit of the wind tunnel. A schematic of the existing nozzle is shown in Figure 5. The inlet height and width is 1 foot to fit the wind tunnel. The exit height is 3 inches.

	Figure 5:  Dimensions of Components

After the nozzle, there will be a straight section so that the transition of the flow from the nozzle to the diffuser is gradual. The dimensions of this section will be a length of two nozzle exit heights, 6 inches, and a height of 3 inches.

3.2 Diffuser Design

The straight section will be connected to the diffuser. The design of the diffuser was based on the ratio of the length of the diffuser (N) to the width (W1) of the diffuser. For a ratio of 6, which would make the diffuser shorter and easier to work with, N is 18 inches for a W1 of 3 inches. The width of 3 inches was determined by the existing nozzle. A schematic of the dimensions of the diffuser is shown in Figure 5.

The performance of the diffuser with the bumps will be tested in the region of transitory stall. With a ratio N/W1 of 6, the range of divergence angles in this region of transitory stall is between 10 and 30 degrees (this is 2), as shown in Figure 3. For this project, the divergence angle settings for the diverging wall will be 5, 10, and 15 degrees (this is ). These three angles were chosen as representative of the range of possible divergence angles with different degrees of flow separation.

3.3 Bump Design

The bumps will be strips of tape which is currently being used in Prof. Breuer's lab for this type of research. The height of the bump is 100 microns and the width is 1 centimeter. The height of the strips must be small enough to prevent flow separation. The length of the bump will be the width of the diffuser which is 1 foot. The bumps will be placed in the diverging wall of the diffuser. The first one will be placed where the flow begins to separate for a given frequency and the rest will be placed alternately upstream and downstream of the first bump.

The number of the bumps as well as the spacing between the bumps will be varied. The number of bumps placed in the diffuser diverging wall will be 1, 3 and 5 bumps. The spacing between the bumps will be the width of the tape strip, twice the width of the tape strip and one and one half the tape strip width (1, 2, and 1.5 cm.)

3.4 Actuator Design

The actuator will introduce the initial disturbance in the flow and enable the bumps. For this project a loudspeaker will be used since it can be operated at the frequencies that will be needed to affect the boundary layer. This frequencies range from 200 to 300 Hz. The loudspeaker will be placed in two different positions. For half of our tests, it will be mounted flush with the straight wall of the diffuser through a hole having the same diameter as the speaker and protected by a wire mesh from the flow. In the other half of our testing, the speaker will be placed a distance of two maximum exit widths (W2) or 16 inches from the exit of the diffuser and the top wall of the diffuser will be replaced with a new uncut wall. It may be interesting to see if the actuator can influence the behavior of the diffuser at a distance. Both speaker locations are shown in Figure 6.

	Figure 6:  Speaker Locations

3.5 Velocity and Frequency Design

For this project, the wind tunnel velocity will be set at 50 m/s. The frequency will be determined from the Strouhal number. This dimensionless factor relates the diffuser inlet height and wind tunnel velocity to the frequency.

4.0 Test Matrix

As before, the test matrix, shown in Figure 7, consists of one half of the tests conducted with the speaker mounted on the wall of the diffuser and the other half of the tests will have the speaker at a distance two times the exit height of the diffuser. Then each set of tests will have a different combination of number of bumps and spacing of bumps.

The testing will be conducted in three phases. For each phase, the flow velocity, the frequency, and the divergence angle, , will be set.

	Figure 7:  Test Matrix

5.0 Equipment and Materials

Following is a list of the materials and equipment required and the status of each.

	Material				Status

Nozzle Available

Plexi-glass On Order

(15 sq. ft. for diffuser)

Wood On Order

(50 ft., 2"X 4" for stand)

Tape Available


10 Pressure Taps Available

Slant Manometer Available

Hot Wire Velocity Probe Available

Pitot Tube Available

Speaker Available

Function Generator Available

Amplifier Available

Voltmeter Available

Safety Headphones Available

Miscellaneous Available

(nails, screws, sealant)

5.0 Data Collection and Analysis

To assess the effectiveness of the bumps in diffuser performance, the experimental data obtained in this project will be compared to the previous research related to delaying diffuser stall and acoustic receptivity. We will compare the coefficient of pressure, defined as the pressure rise normalized by the dynamic pressure, and the divergence angle. The pressure will be measured by 10 pressure taps spaced 2 in. from each other in the diverging wall. The hot wire velocity probe will be used to determine when the flow begins to separate in the diverging wall. Separation of the flow is characterized by the deceleration of the flow. A pitot tube will be placed at the inlet to measure the velocity of the incoming flow.

6.0 Procedure and Schedule

As shown in Figure 8, the background research and the design has been completed. The materials are available or have been ordered. Next term, the diffuser will be built and the test structure will be assembled. The first part of testing will include tests to determine where the flow separates for a given frequency and divergence angle. Then the testing will continue with the placement of different combinations of divergence angle, speaker location, bumps, and bump spacing. The work schedule is given in more detail in Figure 8.

	Figure 8:  Work Schedule for Spring and Fall Terms

7.0 References

1. Chen, D., Z. Sheying, "Control of Separation in Diffusers Using Forced Unsteadiness." Nanjing Aeronautical Institute. AIAA 89-1015, 1989.

2. Oh, David, "Active Control of Airflow Instabilities in 2-D Diffusers." 16.622 Final Report, MIT, 1990.

3. Atkins, S., "Development of an Active Control System to Suppress, Using Forced Unsteadiness, Separation in Low-Speed Diffuser Flow." 16.622 Final Report, MIT, 1990.

4. Kwong, A., Dowling, A.P., "Active Boundary Layer Control in Diffusers." Cambridge University: Cambridge, UK. AIAA-93-3255.

5. Wiegel, M., Wlezien, R.W., "Acoustic Receptivity of Laminar Boundary Layers over Wavy Walls." Illinois Institute of Technology: Chicago, IL. AIAA-93-3280.