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Diploma thesis entitled "Study of the effect of excitation on a circular air jet". Nech.) was accepted as satisfactory in partial fulfillment of the Master of Science in Engineering (Mechanical Engineering) on July 15, 1993. Constant-temperature hot-wire anemometer responses and First Fourier Transform (FFT) analyzer traces were analyzed to obtain an articulated picture of the jet response to the clean.

GENERAL

FREE JET

FLOWIN CIRCULAR TURBULENT FREE JET

COHERENT STRUCTURE ANDPREFERREDMODE

MODEOF FLOWANDLENGTH SCALES

Sufficiently close to the nozzle with a top-hat exit profile (ie, Be/D « 1), the mixing layer of a circular jet is not unlike that of a plane layer. Further downstream, at approx. X '" D, the thickness of the mixing layer becomes comparable to D, and the effect of the azimuthal curvature can therefore no longer be ignored.

JET ANDCONTROLLED EXCITATION

Beand D are thus the two characteristic length scales for the shear layer and beam column modes. They also noted that the turbulence level is highest (with the peak occurring at X/D "2) when the jet is excited in the stable jet column pair mode.

TURBULENCE SUPPRESSIONUSINGCONTROLLED EXCITATION

They also noted that the excitation amplitude should be small but sufficiently above the background turbulence level (free-stream turbulence).

COHERENT STRUCTUREANDAERODYNAMIC NOISE

These facts suggest that mating, whether induced or natural, can produce sound but cannot be the dominant cause of noise in practical jet aircraft. It is possible to excite the jet to enhance the turbulence and hence the noise amplification and also to suppress turbulence and side by side the suppression of noise.

MOTIVATION BEHINDTHE RESEARCH

Some features of denoising are: the spectral range of denoising corresponds to that for turbulence denoising in the noise-producing regions; excitation can produce a net suppression of noise even when the excitation sound is not subtracted; because field noise spectrum is quite broad and without any sharp peaks at f/2, f/4 and so on. Selim [44) studied the flow structures of the circular wedge-shaped jet and recommended investigation with upstream excitation.

SCOPEOF APPLICATION

Selim [44) studied the flow structures of a circular wedge-shaped jet and recommended investigations with upward excitation. Hossain [14) studied the excited shear layer in a circular air jet as an extension of Selim's work and recommended investigations in the developed region and also with surface excitation.

OBJECTIVES

To determine the most appropriate self-preservation variable from the six different variables used by other researchers. To compare the results of the current study with experimental results related to other researchers.

RESEARCH HIGH-LIGHT

Measurements of longitudinal turbulence intensity and mean velocity were made using single hot-wire probes, at the nozzle exit and along the center line a~. The experimental results also compared to the problem under current study, experimental results were made.

INTRODUCTION

He found that the large eddies in the fully turbulent region of the flow are roll-like structures with axes roughly aligned either with the direction of the deformation associated with the mean-velocity gradient or with the. From de/dX it is deduced that the zero speed side entrainment velocity is 3.2% of the free stream velocity.

AXISYMMETRIC JET

They found that the length of the potential core increases with the increase of the Reynolds number and the decay and spreading velocity of the jet decrease with the increase of the Reynolds number. The location of the virtual origin was at X/D = -0.483 upstream for the initially turbulent boundary layer.

EXCITATION IN AXISYMMETRIC JET

The most favorable condition for vortex pairing was determined as a function of excitation Strouhal number (St), Reynolds number (Re) and initial condition of the shear layer, i.e. The mean phase vorticity data showed that the periodicity in the transient of the coherent structures was lost beyond X/D even though the structures are initially organized by controlled excitation in the preferred jet mode.

INTRODUCTION

AXISYMMETRIC TURBULENT MIXING LAYER

The similarity of the velocity distribution in the axisymmetric mixed layer has been well maintained by the observations of Albertson et al. 1, the properties of the axisymmetric mixed layer should not differ from those of the planar mixed layer, provided that Be/D « 1; Be is the output displacement layer.r momentum thickness.

AXISYMMETIDC TURBULENT JET

Husain and Hussain [18] found that the shear layer achieved self-preservation in the range 1.5 < X/D < 4.0 for both laminar and turbulent layers. The analysis of the other experimental results Abramovich 0] suggested a value of 0.097 for C2• which was close to the value of 0.0965 for the data of Albertson et al.

SELF PRESERVATIONPROFILES

From similarity analysis of the equations of motion, it can be shown that Uc ~ 1IX and YO•50 = bj ~ X, which can be written as Uc=C1/X and bj=C2X. Squire and Trouncer [46] used (YO.OI-Y)/(YO.IO-YO.9S) as the self-conserving variable for composite mixing layers, while Rajaratnam and Pani [39] used.

INITIAL AND BOUNDARY CONDITIONS

The turbulence intensity profile u' should increase monotonically as the wall is approached, recalling a maximum essentially at the wall, i.e. at Y :5 0.16; where 6 is the shear thickness of the boundary layer. The momentum velocity tJ;lickness d8/dX is nearly equal to the ratio Ve/Uec', where Ve is the transverse entrainment velocity.

MISCELLANEOUS

Peak and Shear Layer Turbulence Intensity: Peak longitudinal turbulence intensity, up', is defined by the individual maximum value of u' measured in the radial plane. In the second group, a detection element is introduced into the flowing liquid and the turbulence is measured by the change in mechanical, physical or chemical nature occurring in this element.

THE CIRCULARAIR JET SYSTEM

COORDINATE ANDDIMENSIONOF FLOWSYSTEM

To check the asymmetry of the nozzle flow supply, the axial average velocity measurements were performed at three axial locations, namely are within the experimental errors, for any four axial locations.

MEASUREMENT OF MEANANDTURBULENTQUANTITIES

TURBULENCE SIGNALFROMFFT ANALYZER

CALCULATION PROCEDURE

GENERAL

EXIT CONDITIONS

The profiles are compared to the Blasius profile for the flat-plate laminar boundary layer, whose shape factor is about 2.59. The profile at Reynolds number 8.56 x 104 is considered turbulent, whose shape factor is about 1.42.

GENERAL JET RESPONSES

After X/D = 5.0, the growth rate is higher for higher Reynolds number and reaches its maximum value around X/D = 9.0. After X/D= 2.0 the turbulence intensity at the lowest Reynolds number decreases while that at the highest Reynolds number increases.

INTEGRALSCALESANDVARIABLES

Streamwise evolution of terminated momentum thickness eO•10 of unexcited and excited jet at Red= 8.56 x 104 is represented in the figure. Since it is maximum at X/D = 3.0, fluctuating momentum remains almost constant until X/D = 6.0 while mean momentum decreases at a sharp rate until X/D = 8.0; the rate of decrease of mean momentum then slowed beyond the above range.

SELF PRESERVATION OF MIXINGLAYERANDJET

The self-preservation profiles of the unexcited jet are presented in Figs. The effect of the upward excitation on the centerline longitudinal turbulence intensity at Red= 8.56 x 104 is presented in Fig. The excitation effect is strong in the near field of the jet jump.

Similar but more organized turbulence enhancement and suppression is observed in the case of surface excitation, which is presented in Fig.

TURBULENCE SIGNAL ANALYSIS ; ';

At X/D = 5.0, the surface excitation shows earlier and larger turbulence amplification but similar and unstable damping and is presented in Fig. Thus, surface excitation can result in much greater turbulence amplification in the near-field of the jet, but has similar damping properties to unsteady ones. 5.6.34, the downstream crossing reduces the effect of the surface excitation, but still remains iIi a prestigious position than the upstream excitation.

The frequency spectrum of the unexcited jet at four centerline X/D locations is shown in Fig.

GENERAL

CONCLUSIONS

Similar, but more organized, turbulence growth and suppression is observed in the case of surface agitation. Upstream excitation can increase the centerline turbulence intensity by up to 48% and suppress 76% in the near-field of the aircraft. In the near-field of the aircraft, the surface excitation can increase the centerline turbulence intensity by up to 58% and suppress about 46%.

Surface excitation can cause much more turbulence enhancement in the near field of a jet, but has similar but unstable suppression properties.

RECOMMENDATIONS

BASIC PRINCIPLES OF HOT-WIRE ANEOMOMETER

In these systems, the response of the hot wire to a speed fluctuation is modified by its internal heat capacity, which becomes significant for fluctuation frequencies above about fifty cycles per second. The proper setting of the compensation circuit depends on the operating conditions of the hot wire and is usually found by what is called a square wave calibration circuit. In the constant-temperature type of instrument, a feedback circuit keeps the resistance and thus the temperature of the hot wire constant.

In the past, feedback system design faced serious problems to achieve high frequency to match the performance of the best constant current system in very high frequency operation.

PRINCIPLES OF PLUG IN UNITS OF DISA CONSTANTCURRENTHOT-WIRE ANEMOMETER CTA 56COOSYSTEM

The energy input to the hot wire must then completely end up in the air flow; the internal capacitance of the hot wire is no longer important, because its temperature is constant, and therefore this energy input (proportional to the square of the current) is a measure of the instantaneous air velocity. The sampling signal to the average value of the filtered input signal is greater than 50 m/sec. The resulting frequency is directly proportional to the instantaneous value of the filtered input voltage.

The input signal can therefore be integrated by counting at this frequency (5.12 MHz) through the integration period and dividing the result by the duty cycle of the integration.

I TRANSDUCER CALIBRATION

UNCERTAINTY ANALYSIS

Hussain, A.K.M.F. og Clark, A.R., 1981, On the coherent structure of the axisymmetric mixing layer: A flow visualization study, Journal of Fluid Mechanics, bind 104, pp. Hussain, A.K.M.F'.and Husain H.S., Elliptic jets, 1989 , Del I, Characteristics of unexcited and excited jets, Journal of Fluid Mechanics, bind 208, s. C., 1982, Strukturen af de store hvirvler i fuldt udviklede turbulente forskydningsstrømme, Del 1, The pIant jet, .Journal of Fluid Mekanik, bind 118, s.

Zaman, KMBQ and Hussain, AKMF, 1981, Turbulence suppression in free shear flow by controlled excitation, Journal of Fluid Mechanics, vol 103, p.