Experimental study of fluid-structure interactions oon a generic model
Abstract: A new type of test facility is presented which allows theinvestigation of fluid-structure interactions using a genericflexible model. Rather than modelling the complex geometry of aturbomachine blade passage or blade row, this test facilityuses a two dimensional generic bump located in a straightchannel in order to reach a better understanding of the bendingflutter phenomenon. Thus, experimental campaigns are performedto observe and measure surface pressure fluctuations linked tothe interactions of a shock wave with the boundary layer formedover the oscillating structure. The new test facility modifiesan existing wind tunnel featuring a straight rectangular crosssection. The oscillating model used in the study is oftwo-dimensional prismatic shape and has been investigated inprevious studies, from which base case data are available. Inorder to introduce capabilities for the planned fluid-structureexperimental campaigns, a flexible version of the model hasbeen built. It is moulded of polyurethane at defined elasticityand hardness, and actuated by a novel type of fully integratedmechanical oscillating mechanism. A frequency controlled ACservomotor drives this oscillating mechanism. The whole drivetrain is able to produce an oscillation of the model atvariable amplitude and frequency up to 200Hz. At the same time,a one dimensional laser sensor measures precisely the wholemodel displacement through a top optical window. The flow inthe test section can be set at different operating conditions.Time-resolved pressure measurements are performed on theoscillating surfaceusing Kulite fast response transducerscoupled to an adapted long line probe technique. While theinstantaneous models shape is scanned using laser triangulationtechnique through the top window, unsteady Schlierenvisualization measurement are performed using the accessthrough two side windows. Similar coupling is also performedbetween unsteady flexible geometry measurements and unsteadypressure measurements. The mode shapes of this flexible bumpstrongly depend on the excitation frequency. It is consideredthat a first bending mode shape is obtained for reducedfrequencies up to 0.037. However, for reduced frequencieshigher than 0.037, the mode shapes are interpreted as higherharmonic stripe mode shapes. Thus a second order mode shape isreached for reduced frequencies between 0.037 and 0.074, and athird order mode shape is reached for reduced frequenciesbetween 0.074 and 0.294. In this experimental study, the modeloscillates at reduced frequencies from 0.015 to 0.294 attransonic flow condition characterized by an inlet Mach numberMiso1=0.69 and an outlet Mach number Miso2=0.80. Schlieren pictures as well as unsteadypressure repartitions are obtained for this operating flowcondition. The presented unsteady results demonstrate that thephase of shock wave movement towards bump local motion shows adecreasing trend for third bending mode shapes fluctuating withreduced frequencies higher than 0.074. At the pressure tapslocated after the shock wave formation, the phase of pressurefluctuations towards bump local motion present the samedecreasing trend for the same kind of mode shapes. However noconclusion can be drawn for this range of perturbationfrequencies at a non-fluctuating bending mode shape (a modeshape that remains the same in the whole perturbation frequencyrange). In conclusion, a new version of this generic bump willhave to be manufactured in order to perform similar experimentsat a non-rigid first bending mode shape.
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