School of Aerospace, Mechanical & Manufacturing Engineering

AERO2358 Advanced Aerodynamics Compressible Flow Laboratory Demonstration Class Notes

Semester 1, 2015 Time / Date of Lab. Class: 11:00 am to 01:00 pm on Tuesday 28 April 2015.

Location of Class: Room 253.02.002, BE Campus

Due date of Report: Midnight Tuesday 12th May 2015

Return date: Tuesday, 26 May 2015

Estimated time to complete: 4 hours

Note: A Group Report is required, which should be completed by groups consisting of

no less than 3 students and no more than 5 students.

Only a single submission is required for each group.

Introduction Quantitative measurements can be difficult and expensive to perform in high-speed compressible flows. Sensor probes are usually larger and more robust than the probes used in subsonic flows. Probes are intrusive and they generate shock waves, which disturb the flow being observed.

It is possible to obtain qualitative understanding of flows as well as limited quantitative information by using optical methods. Optical methods offer the advantage of being non-obtrusive, i.e. they will not generate additional disturbances in the flow.

Background Theory Schlieren Flow Visualization for Compressible Flow Two of the more common optical methods used in compressible flow are the Schlieren and Shadowgraph techniques. Schlieren is a German word referring to the striations or streaks that appear in the visual results of this technique.

The basis of these optical techniques is the variation of refractive index with density. Variations in the gradient of the gas density introduce refractive index variations which cause the light to be deflected from the undisturbed path. Since the density of a gas depends on the temperature, the Schlieren technique is often used to study natural convection problems. The technique is also used to study compressible flows where localized density variations occur because of shock waves (compression) or expansion waves.

AERO2358 Compressible Flow Laboratory Demonstration Class Notes, Semester 1, 2015

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The following link provides more information and some impressive applications of the Schlieren technique.

http://www.sciencephoto.com/search/searchLogic.html?subtype=newimage&searchstring=August+09&set_id=345&country=67

Toepler Schlieren System The Toepler Schlieren system is often used for compressible aerodynamic flow investigations. The AMRAD apparatus used in this laboratory class utilises this type of system. The basic idea of the Toepler Schlieren system is that, without any flow, a knife-edge is adjusted so that it intercepts a portion of the light which passes through the test section before it reaches the viewing screen (or photographic plate). When the flow is established, any region that causes light to be deflected towards the knife-edge will appear darker (since more light is blocked) while any region that causes light to be deflected away from the knife-edge will appear lighter (since less light is blocked). Additional optical components are required to shape and direct rays from the light source. In the implementation sketched in figure 1, the vertical slit and first concave mirror function to illuminate the test section with parallel rays of light. The second concave mirror focuses the light in the vicinity of the knife-edge. In some configurations a lens may be used instead of a mirror. However the common feature of all systems is the knife-edge.

LightSource

Lens

Concave Mirror

Concave Mirror

ViewingScreen

Knife edge

FLOW

Test Section

Light deflected by positive

density gradientLight deflected

by negative density gradient

VerticalSlit

Light Blocked

Brighter image

Figure 1. Schematic diagram of Toepler Schlieren system.

AERO2358 Compressible Flow Laboratory Demonstration Class Notes, Semester 1, 2015

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The Toepler Schlieren system converts the displacement of the image of the source corresponding to the deflection of the light passing through a particular point in the flow field, into a change in illumination of the image of this point on the viewing screen.

Note that there will be no change in illumination intensity if the density gradient causes the light deflection to be parallel to the knife-edge. This feature allows the knife-edge orientation to be selected to view density gradients in different directions.

Note the differences between the images when the knife-edge is placed perpendicular to the chord and parallel to the chord of the diamond aerofoil shown in figures 2(a) and 2(b) below. With knife-edge perpendicular to the chord the oblique shocks at leading edge are both dark.

With knife-edge parallel to the chord the upper oblique is light and the lower shock is dark.

Figure 2. Effect of knife-edge orientation.

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(a)

(b)

Figure 3. Schlieren renderings of two dimensional jet flow. Simulation of standard schlieren: (a) Horizontal knife-edge (aligned parallel to flow) (b) Vertical knife edge (aligned perpendicular to flow) The effect of parallel and perpendicular knife-edge orientation can also be seen in the simulations of standard schlieren method of two dimensional jet flow in figure 3.

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Colour Schlieren The solid opaque knife edge can be replaced with a transparent filter. The transparent filter performs a similar function to the opaque knife edge but the deflected light is coloured rather than being partially blocked or unblocked by a knife-edge. A coloured image is produced compared to the grey-scale image when a knife-edge is used. Figure 4 shows a filter which is blue that is surrounded by red and green filters. The conventional knife-edge schlieren system used I the AMRAD model could be updated to colour schlieren in the future. However, for now the system still uses a knife-edge.

Figure 4. Transparent colour filter replaces knife-edge for colour schlieren images.

Figure 5. Colour schlieren image of flow around an F-14D model

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Conical flow

In class we have only considered Cartesian two-dimensional (e.g. wedge) flows. The models used in this laboratory class are conical, i.e. axisymmetric. Conical flows are of great practical importance, for example, the flow over the nose of a modern supersonic aircraft and a missile are conical. The notation used for conical flows is shown in figure 6 below.

Important features of conical flows.

cs

Flow

proper

ties

cons

tant a

long r

ay

Conic

al Sh

ock

Body

Streamline

Figure 6. Notation for Conical Flow Conical oblique shock located at vertex Streamline continuously deflects downstream of shock and only becomes parallel to the cone at infinity (in contrast to the 2D wedge). All flow properties (p, T, , V etc.) are constant along rays from vertex (definition of conical flow). Analogous relationship exists between s and c but is different from - -M diagram for 2D wedge. The conical shock flow chart is provided below in figure 8.

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Description of Apparatus

An engineering drawing of the AMRAD W4C miniature supersonic wind tunnel used for the laboratory class is provided in figure 7 below. Note how the rigid contour causes a flexible plate to deform and change the throat area. This is how the Mach number of the facility can be adjusted.

Figure 7 Sectioned engineering drawing showing flow passage of AMRAD W4C.

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Lab Report A laboratory report is required using the headings provided below.

In addition to material presented in this document, information concerning the apparatus will also be provided during the Laboratory Demonstration Class. The image from a video camera will be projected onto a screen during the class, to provide an overview of the optics and other components of the facility.

Mere reproduction of diagrams or text from these Laboratory Demonstration Class notes will NOT earn marks.

1) Introduction Background information, including: quantitative measurements in compressible flows; problems with using sensors; advantages of optical methods.

2) Apparatus (a) Sketch and briefly describe the Toepler Schlieren system implementation, i.e. the optical

paths and position of the knife-edge etc. (Do not just copy and paste the diagram on page 2 of these Laboratory Demonstration Class notes).

(b) Describe how a colour Schlieren system could be implemented into the facility. (c) Sketch the airline (i.e. the flow passage seen by the air flow) of the AMRAD W4C and

explain how the Mach number adjustment knob is used to adjust the Mach number of the exit flow.

3) Results (a) There are a number of simple shapes (e.g. cone, blunt nose and sphere) which will be

clamped into the working section. The flow features around these bodies will be explored and at different Mach numbers. Discuss these features with respect to the theory you have covered in compressible flow.

(b) For the flow over the cone, explain why the conical shock appears darker coloured above the cone and lighter coloured below the cone.

(c) Depending on image quality and other factors, each group of students will be designated a particular case (Mach number) for the flow over the cone. The images for each case will be posted on the AERO2358 Blackboard site.

(d) For the case that your group has been designated, estimate the Mach number of the flow using the Conical Shock Chart provided in figure 5 below. You will need to estimate both the cone angle and the Mach angle from the image you have been designated. You must also make a comparison between the expected Mach number from the AMRAD knob setting and the estimate from angle measurement, and attempt to explain any significant differences.

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Lab Report Assessment Breakdown Attendance 10%

Introduction 10%

Apparatus

Schlieren 6%

Colour schlieren 6%

Tunnel Airline 6%

Mach No. Knob 6%

Results

Flow over sphere 4%

Flow over rod 4%

Flow over cone 4%

Explain dark/light 8%

Cone Analysis 16%

Conclusions 10%

Style / Appearance 10%

TOTAL 100%

AERO2358 Compressible Flow Laboratory Demonstration Class Notes, Semester 1, 2015

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Figure 8 Conical Shock Chart

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Figure 8 Conical Shock Chart (continued)