Strojniški vestnik - Journal of Mechanical Engineering 60(2014)3, 147-157 © 2014 Journal of Mechanical Engineering. All rights reserved. D0l:10.5545/sv-jme.2013.1462 Original Scientific Paper Received for review: 2013-09-26 Received revised form: 2013-10-13 Accepted for publication: 2013-12-03 The Experimental Analysis of Cavitating Structure Fluctuations and Pressure Pulsations in the Cavitation Station Ignacijo Bilus1* - Gorazd Bombek1 - Marko Hocevar2 - Branko Sirok2 - Tine Cencic2 - Martin Petkovsek2 1 University of Maribor, Faculty of Mechanical Engineering, Slovenia 2 University of Ljubljana, Faculty of Mechanical Engineering, Slovenia The experimental analysis of unsteady cavitating flow has been performed to compare the static pressure dynamics and cavitation cloud structure dynamics. The analysis of the unsteady cavitation flow field was performed in the wake of bluff body in the laboratory cavitation station. The pressure oscillations were measured downstream of the bluff body with a recessed installation of the pressure sensor. The cavitation cloud structure dynamics was visualized using high speed camera. Pressure and image acquisition was performed simultaneously. The results of both measurements were analyzed in low frequency and high frequency intervals. The low frequency analysis of both pressure and cloud structure oscillations was performed in the interval from 0 to 1000 Hz. The high frequency analysis of the pressure fluctuations was performed with band pass filtering from 300 to 400 kHz and amplitude demodulation. Comparison of the static pressure and cavitation cloud structures fluctuations caused by cavitation cloud shedding in the wake of bluff body showed similarity between both signals. Two distinct frequencies of flow oscillations were recognized and the influence of cavitation number on the strength of pressure and cloud structure oscillations was quantified. The amplitude demodulation method was used to show and discuss the connection between the low and high frequency pressure oscillations. Keywords: cavitation, experiment, bluff body 0 INTRODUCTION Cavitation is the formation and then immediate implosion of cavities in a liquid that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure or velocity. Cavitation can cause different undesirable effects, such as performance loss, damage by pitting and erosion, structure vibrations and noise generation in the machinery. Cavitation can take different forms, depending on hydrodynamic conditions. Stable cavities, defined as sheet cavitation, may develop in the low pressure regions attached on solid walls. If sheet cavity increases over the critical size, periodic fluctuations of cavity appear. These oscillations are accompanied with adverse pressure gradients and downstream shedding of clouds. The cavitation cloud shedding in the wake of cylindrical bluff body is the subject of present study. Many numerical and experimental studies on cavitating flows have been performed. The numerical studies, performed nowdays mainly focus either on mathematical and physical model development [1], its optimization [2] or on cavitation erosion prediction [3]. The experimental studies have longer history. Pioneer work for the case of cavitation flow visualization around a hydrofoil was carried out by Wade and Acosta [4] in 1966. The major aim of study was to measure lift and drag coefficients of the hydrofoil with and without the presence of cavity oscillations. Several studies were performed where frequency of cavitation clouds shedding was analysed. Among them are Kjeldsen et al. [5] who classified the types and length of cavitation by a combination of angle of attack and cavitation coefficient , and compared the range of periodical shedding of the cloud cavitation to the Strouhal number. The experiments performed by George et al. [6] revealed that oscillations of cavitation clouds possess similar range of Strouhal numbers. Cavitation clouds have complex structure and behavior. Measurements performed by Kubota et al. [7] showed that the cloud cavity consisted of a large-scale vortex and cluster of small vapor bubbles located in the center of the vortex. Such complexity of cavitation clouds is reflected in periodic nature and variability of pressure oscillations. Pressure oscillations were measured using piezo-electric transmitters and their classification into local and global pulses was performed by Reisman and Brennen [8]. The development of modern visualization techniques resulted in detailed shedding phenomena and frequency content investigation performed on a case of spherical bluff body by Brandner et al. [9]. Strongly connected to this paper are studies about simultaneous pressure measurements and visualization of cavitation cloud structure using high speed imaging. Reisman et al. [10] studied shock waves in cloud cavitation using high speed visualization and *Corr. Author's Address: University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, 2000 Maribor, Slovenia, ignacijo.bilus@um.si 147 Strojniški vestnik - Journal of Mechanical Engineering 60(2014)3, 172-178 pressure measurements. Langa et al. [11] performed simultaneous visualization of cavitation cloud structure and pressure fluctuations in the cavitation tunnel. Visualization of cavitation cloud structure was performed with a high speed camera while pressure fluctuations were measured by an array of sensors in a thin piezoelectric membrane. Good agreement was observed among the measured variables. Few other studies of simultaneous visualization of cavitation cloud structure exist. More often cavitation cloud structures are compared to cavitation pit damage Keil et al. [12], Van Terwisga et al. [13], Heath et al. [14]. Quantification of cavitation by visualization with several approaches was reported in the literature. Quantification methods include counting bubbles, estimation of the area or volume of cavitation and measurements of average greyness intensity of cavitation clouds. Methods also differ in the arrangement of cameras and illumination. Leppinen and Dalziel [15] reported on the case of non overlapping travelling cavitation - bubbly two phase flows using background illumination. Here the camera was used to measure the attenuation of light as it passes through a bubbly flow, and this attenuation was related to the void fraction. Makiharju et al. [16] measured the void fraction distribution in gas-liquid flows using a two-dimensional X-ray densitometry system. Void fraction was measured based on transmittance of X rays through a test specimen, and sources of measurement uncertainty such as X-ray scatter, image distortion, veiling glare and beam hardening were considered. For very low void fractions, where individual bubbles were distinguishable, void fraction was also estimated by visualization method. Cavitation bubbles border detection and size estimation using two ellipse principal axes were performed. A limited comparison of X-ray void fraction measurement with estimation based on visualization method shows a fair agreement between both methods. The X-ray measurement method was compared to visualization by Bauer et al. [17] as well. Visualization was performed by diffuse illumination from the side of the test specimen, while experimental results showed superiority of X-ray measurement. Visualization was also used by Maurus et al. [18] in the case of subcooled flow boiling to estimate void fraction. Authors arranged illumination from the side and due to the spherical shape of the bubbles, light was reflected at the phase boundary layer between vapor and liquid in diverse directions. Thus, void fraction estimation analysis included several steps to measure the number and size of bubbles. Illumination from the side using a thin light sheet was also applied by Iyer and Ceccio [19] for measuring the influence of developed cavitation on the flow of a turbulent shear layer. Fig. 1. Experimental setup, cavitation station with cavitation test section, detail shows bluff body and pressure transducer installation 148 Biluš, I. - Bombek, G. - Hočevar, M. - Širok, B. - Cenčič, T. - Petkovšek, M. Strojniski vestnik - Journal of Mechanical Engineering 60(2014)3, 147-157 In the present study we investigate cavitation around a bluff body with simultaneous visualization of cavitation cloud structure and pressure measurements and compare measured variables in the area of the wake. 1 THE EXPERIMENTAL SETUP The experimental setup in Fig. 1 was used for the experiment. It consists of cavitation station with cavitation test section and measurement equipment. It is more thoroughly discussed in the following subsections. 1.1 Cavitation Station Cavitation station is shown in Fig. 1. The main elements of cavitation station are water tank, main circulation pump, vacuum pump, cavitation test section and measurement equipment for cavitation station operational point regulation. The quantity of water in the cavitation station was approximately 1 m3. It was driven by a radial circulation pump. Unprepared tap water was used for the experiments. Water quality was not measured, but we replaced the water in the cavitation station prior to measurements with fresh tap water. Operating pressure in the system was set by a vacuum pump through a three way valve at the top of the water tank. Measurement equipment for cavitation station operating point selection comprised of pressure and volume flow rate measurement equipment. Operating pressure was measured with absolute pressure transmitter ABB 2600T Series 264 VS. Pressure drop on the test section was measured with absolute pressure transmitter ABB 2600T Series 264DS. Four pressure taps were located upstream and downstream of the test section. The uncertainty of the operating pressure measurement was estimated to ±0.5% of measured value. Volumetric flow rate was measured with electromagnetic flow meter ABB COPA-XL DL43F DN 125/PN 16. The measurement uncertainty of volumetric flow rate was up to ±1% of measured value. The experiment was performed with water temperature close to ambient temperature (T = 24 °C). Water temperature was measured with Pt100 thermometer mounted inside the water tank and connected to Agilent 34970A data acquisition unit. Temperature measurement uncertainty was estimated to ±0.5 °C. 1.2 Test Section Cavitation station test section is shown in Fig. 1. The test section dimensions were 50 mm (width) x 10 mm (height) x 800 mm (length). Pressure pulsations were generated by a bluff body with diameter of 16 mm. The height of the bluff body was the same as the height of the test section. The bluff body was manufactured from hard rubber and was compressed and held in place by wall friction. No cavitation was observed near the junction of the bluff body and test section wall. The bluff body was mounted 50 mm upstream from the pressure transmitter. Measurement equipment consisted of a fast pressure transmitter and a high speed camera. Pressure oscillations were measured with PCB Piezotronics pressure transmitter type 111A26 with frequency range from 0.01 Hz to 400 kHz and dynamic pressure range of 34.5 bar. The transmitter was mounted to the wall of the cavitation test section as shown in Fig. 1. The pressure transmitter was mounted in an insert, which was flush mounted to the test section lower wall. The transmitter was connected to the test section through a hole 3 mm in length and 2 mm in diameter and a cavity 2 mm in length and 5 mm in diameter, as shown in Fig. 1. The transmitter was protected against cavitation erosion with the above mentioned installation installation. Power to the transmitter was supplied using PCB Piezotronics 480C02 signal conditioner. The signal conditioner has a frequency range from 0.05 to 500 kHz. Pressure was acquired with 16 bit data acquisition board national Instruments NI-USB 6351 with 1.25 MHz sample rate. The convenient roll-off of the signal conditioner and selected frequency of acquisition enabled operation without aliasing filters. The duration of acquisition was 2 s. Data was stored to the disk immediately after the acquisition. Image sequences were acquired using high speed camera Fastec Hispec4 with frequency of acquisition of 10 kHz. Image resolution was 416x272, image depth 8 bit and shutter time was 30 ^s. In every operational point 10000 images were acquired in a sequence, amounting to acquisition time 1 s. For all series of images, obtained by visualization method, camera settings for brightness and contrast were constant and equal. The lens used was manual Nikkor 50 mm, aperture F1:1.2. The camera was mounted perpendicular to the cavitation station wall at a distance of 1 m. Illumination was continuously provided using 8 CREE XM-L T6 LED lights. These were mounted to the side of cavitation test section at The Experimental Analysis of Cavitating Structure Fluctuations and Pressure Pulsations in the Cavitation Station 149 Strojniški vestnik ■ Journal of Mechanical Engineering 60(2014)3, 147-157 a distance 20 cm parallel to the cavitation station test section wall. A 70 mm cylindrical lens was attached to LED lights. It was used to focus illumination on a light sheet with thickness of approximately the same size as the height of water above the pressure transmitter. The entire distance from the bluff body to the pressure transmitter was uniformly illuminated. Such installation also prevented any unwanted reflections and provided very strong illumination required by the short shutter time. Pressure and image acquisition was synchronized with the trigger from an electronic synchronization device. The trigger started the acquisition; later acquisition timing was performed using internal clocks of the data acquisition board and the camera. 1.3 Image Processing and Void Fraction Estimation The high speed camera acquired series of eight-bit greyscale images that were showing cavitation inside the cavitation test section. For image post-processing image with pixels can be presented as a matrix with elements. Eight-bit resolution gives 256 levels of greyness A(i,j, n), which the matrix element can occupy (0 for black pixel and 255 for white pixel) A(i,j,n) e {0,1,..., 255}. Each image is thus presented as a matrix: did not take into consideration the shape of cavitation clouds, which has a major influence on the cavitation cloud void fraction. While the above procedure provided useful results, other measurement methods give better estimation of void fraction. Among them is x-ray computed tomography measurement [16] or time resolved 2D X-ray densitometry [21] method. A rectangular region of interest was used as shown in Fig. 2. We used the size of ROI 65^65 pixels which corresponds to 10.4^10.4 mm. The pressure transducer hole was located in the same position as the center of ROI. All sets of images were analyzed with ROI of the same size and position. bluff body cavitation cloud \ / pressure transmitter region of interest Fig. 2. The region of interest (ROI) in the wake of bluff body Image (n ) = A(1,1, n) A(1, J, n) A(1,1, n) A(I, J, n) (1) For the evaluation of brightness (i of a region of interest we averaged brightness of the pixels in the region of interest (ROI) [20]: J j n). (2) Although the exact relationship between the of cavitation clouds void fraction A and average ROI brightness i cannot be established with single BW camera and present image acquisition configuration, we assumed that cavitation cloud void fraction A is proportional to the brightness i in a ROI corresponding to a cavitation cloud: A(n)