Static flow resistivity

Most textiles that are used for acoustic purposes show open porosity due to many interconnected pores or voids inside. The acoustic performance of a porous textile is mainly determined by its (air) flow resistivity, which is an intrinsic property of the textile and is a measure of how easily air can enter and pass through a porous textile material (Cox and D Antonio, 2009). Flow resistivity, also known as static flow resistivity, is related to acoustical properties and plays a critical role in the calculation of many intrinsic acoustic properties of porous textiles, such as the characteristic impedance, the propagation constant, and the sound absorption coefficient. In the S.l. Unit system, flow resistivity is quoted in units of Nsm" and is defined as the unit-thickness specific flow resistance o (Morfey, 2001),... 

Flow resistance is also known as static flow resistance and is the ratio of the pressure drop across a porous element to the volume velocity flowing through it under conditions of steady low speed flow. The flow resistance is almost independent of the volume velocity at low speeds. However, flow resistance is dependent on the acoustic frequency. Dynamic specific flow resistance of a thin (compared to acoustic wavelengths) porous textile layer is the real part of the complex specific flow impedance at a specified frequency, which is defined as the complex ratio of the pressure drop across the layer to the relative face velocity through the layer. When the frequency tends to zero, the dynamic specific flow resistance varies little with frequency, so it is almost equal to the static flow resistance. In the international standard, flow resistance is defined as the real part of the ratio between the pressure drop and the flow velocity through a layer of material of unit thickness (ISO 9053,1991). Flow resistance characterizes a layer of specified thickness, whereas flow resistivity characterizes a bulk material in terms of resistance per unit thickness. 

There are many acoustical methods proposed for measuring flow resistivity (Delany and Bazley, 1971 Smith and Parott, 1983). A method that uses a standard impedance tube directly to measure the static flow resistivity without any additional requirements to tube modification or sensor location change is described in ISO Standard, 10534-2 (1998) and by Tao et al. (2015). In the method, the specific acoustic impedance on the front surface of the test specimen is measured first by using the traditional transfer function method with the test specimen being placed against and with a known interval to the rigid termination, and then the characteristic impedance, the propagation constant, and the static flow resistivity are calculated based on the obtained impedance transfer functions. 

By substituting the calculated k into Eq. (6.19), the characteristic impedance of the test specimen can be obtained For porons materials, the static flow resistivity can be calcnlated by Eq. (6.21) (Doutres et al., 2010) ... 

In practical applications, the lower limit of the measurement frequency in ISO 10534.2 is 63 Hz and it is impossible to generate incident sound waves at zero Hz as required in Eq. (6.22). Fortunately, the flow resistivity is not sensitive to frequency when the measurement frequency is sufficiently low (Ren and Jacobsen, 1993), so the determined value of static flow resistivity is expected to be acceptable when the measurement frequency is around a few hundred Hz. 

Picard, M.A., Solana, R, Urchuegfa, J.F., 1998. A method of measuring the dynamic flow resistance and the acoustic measurement of the effective static flow resistance in stratified rockwool samples. J. Sound Vib. 216 (3), 495-505. 

In the JCA model, the pororrs material has to be characterized by its porosity ((>, its static flow resistivity a, its tortuosity a , its viscous and thermal charaeteristie lengths A and A, its bulk density and its meehanieal properties (Young s modulus, poisson ratio and loss factor). In practice, these parameters need to be determined using direct and/or indirect techniques. A detailed description of these parameters and their determination methods can be found in the hterature [1,3, and 7]. In the current work, a Nitrogen pycnometer is used for the measurement of the porosity [7] the flow resistivity is measured following ASTM-C522 the Ultrasound technique is used to measure the tortuosity [8] and an inverse method is used to estimate the viscous and thermal characteristics lengths from the normal incidence absorption tests [7,8]. 

Performance Curves. Pan manufacturers furnish fan performance curves for each type fan available. These are typically based on 61 m/s (12,000 ft/min) tip speed and 1.20 kg/m (0.075 lb /ft ) density. To select a fan for a specific duty requires knowledge of the flow, static pressure resistance, and density of the actual operating conditions. Usually the fan diameter is known as well as some idea of operating speed a 61 m/s tip speed can often be assumed. 

Finally, several attempts have been made to develop an absolute molar mass detector based on osmotic pressure measurements. Commercially available membrane osmometers are designed for static measurements, and the cell design with a flat membrane is not suited for continuous flow operation. Different from the conventional design, Yau developed a detector which measures the flow resistance of a column caused by osmotic swelling and deswelling of soft gel particles used for the packing (see Fig. 12) [65,78]. With a microbore gel column, a... 

Total lung flow resistance Static lung compliance Flow resistance of airways... 

Figures 322(a) and (b) show again the conditions in a pelleting machine with concave die. Figure 322(a) depicts the mechanisms of compression and extrusion in the work area , the material volume wedged in between the press roller and die. Figure 322 explains the phenomena. Feed deposited in a layer on the die is pulled into the space between the roller and die and compressed. Neither the roll force nor the force from the die resisting extrusion (flow) are constant. The roll force increases with progressing densification while the flow-resisting force remains constant until a threshold pressure, defined by the static friction in the die holes, is surpassed. After extrusion (movement in the die holes) has started, both the resisting and the roll forces decrease.

Two notable approaches have been used to include the role of pore constrictions in the pressure gradient required to drive lamellae through porous media. Falls et al. (48) added a viscous resistance that accounted for pore constrictions and that acted in series with the straight-tube flow resistance of Hirasaki and Lawson (18). Prieditis (41) and Rossen (42—44, 46) computed the static curvature resistance to the movement of a single bubble and also trains of bubbles through a variety of constricted geometries. Rossen considered the role of bubble compressibility (43), asymmetric lamella shapes (44), and stationary lamellae (46) on foam mobilization. 

The application of forced airflow is necessary for drying products in static beds, which form a comparatively large flow resistance in the bed. Such products include grains and hay. One solution is to build a solar room dryer [13] as shown in Figure 14.7a. The grains to be dried are placed as a bed 1 on a perforated flooring. Collectors 2 are located on the southern wall and the roof of the building. The air warmed... 

The flow resistance of the mixer can impose excessive stresses on the mixer itself. Following element folding, in an angled plate static mixer where the welded elements proved unable to withstand the loads applied in a melt cooling application, it was replaced by a single piece element machined from a solid steel blank. This proved suitable for the task (Figure 11.5). 

By far, the most often cited work is the early paper by Outmans (1963), which applied differential-equation-based filtration methods developed in chemical engineering to static and dynamic invasion in the borehole. In this single-phase flow study, where lineal flow was assumed and the applied differential pressure was completely supported by the mudcake, Outmans derived the well known Vt law, subject to the further proviso of cake incompressibility. (The effects of cake nonlinearity and compaction can be important over time e.g., see Figure 14-7.) Thus, the Vt law cannot be used when the net flow resistance offered by the formation is comparable to that of the mudcake (e.g., thin muds in permeable formations, or thick muds in very impermeable rocks). Also, the law does not apply to slimholes, where the radial geometry is important. Finally, the Vt law does not generally apply to reservoirs with two-phase, immiscible flow, or miscible flow, or both. Only under these restrictive assumptions does Outmans correctly derived law hold. 

Since the term Apfr given by Ergun correlation is the contribution of fluid flow resistance in the total static pressure difference Ap, we can write... 

Knoop developed an accepted method of measuring abrasive hardness using a diamond indenter of pyramidal shape and forcing it into the material to be evaluated with a fixed, often 100-g, load. The depth of penetration is then determined from the length and width of the indentation produced. Unlike WoodeU s method, Knoop values are static and primarily measure resistance to plastic flow and surface deformation. Variables such as load, temperature, and environment, which affect determination of hardness by the Knoop procedure, have been examined in detail (9). 

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