Gap Flow Research Articles

Improvement of the efficiency for rim-driven thrusters through acceleration of gap flow

Efficiency optimization of the rim-driven thruster (RDT) has emerged as a prominent research topic in recent years. In this work, the Reynolds-averaged Navier-Stokes (RANS) solver is employed for simulating open water performance of RDTs, aiming at improving the efficiency of RDTs. Specifically, a series of RDTs are obtained by modifying the Ka4-70 propeller with a 19A duct, and their thrust and torque components, as well as efficiency and gap flow, are thoroughly examined. The investigation commences by exploring the impact of different rim sizes on RDT efficiency. A comparison is conducted between the open water performance of RDTs and the ducted propeller. Subsequently, a case with relatively high efficiency is identified, serving as a basis for the development of a series of modified RDTs equipped with acceleration devices. These devices facilitate accelerated axial flow in the gap, thereby enhancing the thrust generated by the duct, while simultaneously reducing the torque exerted on the rim through accelerated circumferential gap flow. Comparative analysis reveals that the improved RDTs can achieve an open water efficiency increase of up to 10.9% when the advance coefficient is set to 0.6, resulting in an efficiency value of 0.519. The findings presented in this paper introduce a novel direction for optimizing the efficiency of RDTs, contributing to the advancement of this field of study.

Non-equilibrium plasma assisted ignition characteristics in premixed ethylene-air flow

Reliable and stable ignition under lean conditions is essential for safe operation of the engine. Nanosecond pulsed discharge non-equilibrium plasma assisted ignition characteristics of premixed ethylene-air flow in an advective combustion chamber were investigated. The effects of the equivalence ratio, discharge gap distance, flow velocity, discharge frequency or inter-pulse time, and pulse number were quantified in terms of ignition probability. Shadow images of ignition kernel development were captured and used to extracted the averaged kernel projected area. The results indicated that increasing the equivalence ratio, a higher flow velocity, a wider discharge gap distance, and a larger number of pulses are all conducive to the increasing of ignition probability via inducing a larger initial kernel. Increasing inter-pulse time has a non-monotonic effect on ignition probability for multiple nanosecond pulsed discharges ignition. As the inter-pulse time decreases, when neighboring kernel boundaries happen to overlap each other, the partially-coupled regime shows a higher ignition probability. Longer or shorter inter-pulse time both cause the decrease in ignition probability. The shortest inter-pulse time shown as the fully-coupled regime is the most favorable for ignition with the highest ignition probability. A method is proposed to estimate the critical frequency at which partially-coupled regime transitions to fully-coupled regime by 95% of the asymptotic time of flame development time.

Examining homogeneity of dielectric barrier discharge using dispersion normalization and local temperature difference methods

The homogeneous discharge mode in dielectric barrier discharge (DBD) has sparked wide interest in recent years. Assessing the dielectric barrier discharge homogeneity value (DHV) offers value in the industry. The current method of assessing DHV involves distinguishing between the filamentous and homogeneous modes of discharge but struggles to qualitatively analyze it. This study proposes a method for evaluating DHV that involves using the dispersion normalization and the local temperature difference methods. The values of such parameters as the voltage, frequency, air gap, and gas flow were tested to verify the validity and generality of the proposed method. The results of experiments revealed that the dispersion normalization method uses information on the luminosity of the discharge image to assess the homogeneity of DBD under different conditions. The temperature difference in the area of discharge was also calculated to assess the homogeneity of DBD, and the results showed that it is useful to this end. Moreover, the dispersion coefficient and the difference in local temperature were found to be inversely proportional to DHV. The proposed method can thus be used to identify variations in DHV under different conditions, and provides a basis for its characterization.

Flow-induced vibrations of elastically coupled tandem cylinders

We numerically study the transverse flow-induced vibration (FIV) of elastically coupled tandem cylinders at Reynolds number $100$ , using an in-house immersed boundary method-based solver in two-dimensional coordinates. While several previous studies considered tandem cylinders coupled through flow between them, a hitherto unexplored elastic coupling with fluid flow between them significantly influences FIV. We consider a wide range of gap ratio, reduced velocity, an equal mass ratio of both cylinders and zero damping. A systematic comparison between the classic elastically mounted tandem cylinders and elastically coupled cylinders is presented. The latter configuration exhibits two vibration modes, in-phase and out-of-phase, with corresponding natural frequencies approaching the Strouhal frequency of the system. We quantify variation of the following output variables with reduced velocity and gap ratios: cylinders’ displacement; fluid forces; amplitude spectral density of displacement and force signals; phase characteristics; energy harvesting potential; and discuss the wake characteristics using flow separation, pressure distribution, gap flow quantification, and dynamic mode decomposition characterization. The FIV response is classified into several regimes: initial desynchronization with and without gap vortices; final desynchronization; mixed mode; initial branch; lock-in; upper and lower branch; wake-induced vibration; galloping. We draw upon similarities of computed FIV characteristics with those of an isolated cylinder, in which the lower branch exhibits larger a amplitude than the upper branch. The elastically coupled cylinders show a galloping response similar to an isolated D-section cylinder. By invoking the elastic coupling, we demonstrate FIV suppression and augmentation for in-phase and out-of-phase systems. Our calculations show larger energy harvesting potential at reduced cost for elastically coupled cylinders.

Effect of spacing ratios on coupling mechanism of three tandem cylinders in planar shear flow

Using four-step semi-implicit characteristic-based split method, flow-induced vibrations of three tandem cylinders in planar shear flow are numerically simulated at Re = 100. The vibration responses are more complicated than that of a single cylinder due to the gap development and mutual interference with each other. The spacing ratio (L/D) between three tandem cylinders of equal size ranges from 2.5 to 5.5, with a mass ratio (mr) of 2.0 attributed to each one. The reduced velocity (Ur) spans from 3 to 30, and the shear ratio (k) varies as 0, 0.05 and 0.1 respectively. The numerical results show that spacing ratio and shear ratio have an influence on the dynamic responses, performances of frequency, phase and energy for three tandem cylinders. The vibration responses of each cylinder can be clearly divided in three major branches, which first increase then decrease with increasing Ur. The gap flow inside cylinders at small L/D heightens the probability of vortex merging, intensifying mutual interference and consequently amplifying the amplitude. It is worth noting that as k increases to 0.1, downstream cylinders exhibit different vibration responses at L/D ≥ 4.0. Specifically, a secondary increase occurs after Ur > 18 at L/D = 4.0, and the amplitude is completely suppressed after Ur > 25 at L/D = 5.5. With the help of other performances, the differences in vibration responses of three tandem cylinders at different k and L/D are revealed.

Impact of interfacial tension on oil-water flow in a narrow gap

Numerous researchers have examined the co-current flow of oil-water and aqueous solutions containing polymers and surfactants in thin gaps for oil recovery. While some have focused on charges and forces at the interfaces of oil-surfactant solutions during flow. The study of flow structures, interface behavior, and relative permeabilities of oil and aqueous phases of surfactant flow through thin gaps has been less explored. For the first time, this research aims to comprehensively investigate the flow of oil-water and oil-surfactant solutions through a thin gap (Hele-Shaw cell) with a particular focus on the impact of sodium dodecyl sulfate (SDS). The experiments reveal that SDS forms an emulsion near the oil-water interface, capturing oil droplets and enabling their flow along with the SDS solution. Microscopic studies confirm this, showing that when SDS contacts oil, it creates channels through the oil phase, leading to the accumulation and division of oil into small round-shaped droplets, resulting in an oil-in-water emulsion. The addition of SDS to the injecting water significantly enhances relative permeabilities, leading to a remarkable 90% increase in oil recovery from the cell. The research suggests that the optimal SDS concentration range for maximum oil recovery is between 1.5 and 2 wt%, as it achieves the minimum interfacial tension between oil and water.

Experimental Study on Flow Structure Characteristics of Gap Flow Boundary Layer Based on PIV

The boundary layer is the main source of frictional resistance in gap flow, and the study of the flow structure characteristics of the gap flow boundary layer is of great significance for the study of gap flow theory. In this study, the PIV technique was utilized to experimentally investigate the gap flow boundary layers with Reynolds numbers of 16,587–56,870 and gap ratios of 0.6–0.8. The characteristics of the wall friction velocity, the boundary layer thickness, and the wall function of the gap flow boundary layer were analyzed, and the influences of the mean velocity of the gap flow and the gap ratio on the flow structure characteristics of the boundary layer were explored. The results show that using PIV to measure the velocity profile in the viscous sub-layer to solve for the wall friction velocity had good precision. The boundary layer thickness was inversely proportional to the mean velocity of the gap flow and the gap ratio. The wall functions of the boundary layer were as follows: in the viscous sub-layer (y+< 5.5), u+=y+; in the transition layer (5.5 <y+< 26), u+=10.071tanh0.071y+; and in the logarithmic layer (y+> 26), u+=2.78lny++3.8. The thickness of the logarithmic layer was proportional to the mean velocity of the gap flow and inversely proportional to the gap ratio. The inner region of the boundary layer extended to y< 0.18δ or y< 0.13(h/2).

The evolution of print defects in inkjet printers at elevated print gap height

Image uniformity is a key requirement for inkjet printers. When the gap between the print-face and media is large, one of the main causes of non-uniformity is the misplacement of droplets due to air-flow instabilities. Here, we characterise the onset of printed image non-uniformity by measuring the optical density variation of images printed with varying numbers of active nozzles. The optical density data are obtained by scanning printed images, which are corrected to remove any scanner-related effects. Several transects across the image are extracted, the peak spatial frequency is computed using Fourier transform analysis, and the results from these different transects are averaged. The amplitude of the main frequency peak in image density variation exhibits the same onset behaviour as seen in numerical simulations of the fluid flow in the printhead-media gap. Using numerical simulation, we extend the range of the study to estimate the operating envelope for the printing system.

Eccentric drop of a control rod in the guide tube with annular gap flow

The drop time of control rod is essential for safety analysis of nuclear power plant. The motion of control rod can be characterized by an annular gap flow with large blockage ratio. In this study, we studied the effects of the eccentricity between the control rod and the guide tube on the annular gap flow, and further on the drop time. By considering the added mass and the upward flow of fluid in the annular gap, we developed a theoretical model for computing the drop time of eccentric rod. In the proposed model, particularly, the annular gap flow velocity was obtained from full-scale computational fluid simulations with respect to blockage ratio and eccentricity. The simulation and experiment results agree well on the drop time, the histories of velocity and acceleration, and the hydrodynamic characteristics. Results show that the drop time increases drastically when blockage ratio α increases from 0.83 to 0.91, and the eccentricity effect on drop time is negligible when the blockage ratio is small (e.g., α < 0.83). Nevertheless, for large blockage ratio (α ≥ 0.83), the drop time increases significantly for eccentric rod drop, especially for a lighter control rod. The present study provides a general method for analysis of control rod drop, which is critical for the nuclear reactor design.

Evaporation of Leidenfrost droplets on microtextured substrates

A thin thermally insulating vapor layer beneath a levitating Leidenfrost droplet adversely affects the heat transfer from the underlying hot substrate during cooling applications. In this work, we present a theoretical model to determine the total heat transfer to a Leidenfrost droplet on microtextured substrates, taking into account the curved shape of the liquid-vapor interface. The profile of the vapor gap and vapor flow field beneath a Leidenfrost droplet on a microtextured surface is shown to be dependent on the substrate morphology. The shape of the liquid-vapor interface beneath the Leidenfrost droplet, in turn, influences the rate of evaporation on a microtextured substrate. We determine the variation of the minimum and maximum vapor gap over micro-pillared surfaces with different substrate permeability for various droplet volumes and wall superheat. The heat transferred to the droplet is via conduction across the vapor gap beneath the droplet and convection from the ambient air around the droplet. We determine the total evaporation time of a Leidenfrost droplet over a micro-textured substrate using the curved interface model and the conventional flat vapor gap interface model and compare with that obtained from experiments. The overprediction of the average rate of evaporation of a Leidenfrost droplet by the flat interface model ranges from ∼59 % for tall and sparse pillars (marked by high substrate permeability) to ∼ 29 % for short pillars (with low substrate permeability). We show that the average rate of evaporation of the LF droplet obtained using the curved interface model agrees reasonably (within 9%–23 %) with that observed in the experiments.

Research on the Hydraulic Excitation Characteristics of the Top Cover Caused by the Radial Installation Deviation of the Seal of a 1GW Francis Turbine

The radial installation deviation of the turbine runner will change the gap flow between the upper crown and the lower ring seal, which will affect the radial force of the runner and the hydraulic excitation characteristics of the top cover. This research focuses on the 1GW Francis turbine on the right bank of the Baihetan hydropower station. The pressure distribution along the circumference of the top cover was analyzed, and the effects of deviations on the specific generation of hydraulic excitation forces were studied. This research shows that the increase in radial deviation will slightly reduce the output and efficiency, and the radial force on the runner increases parabolically. When the radial deviation is 1.5 mm, the radial force is 5.9 times higher compared to the case without any deviation, and the radius of the fitting circle of the radial force behavior trajectory increases with the increase in radial deviation. In addition, the radial deviation has little effect on the internal flow of the runner and the pressure distribution in the upper crown chamber. The dominant frequency components at the upstream monitoring points include fn, 15 fn, 24 fn, and 30 fn. The dominant frequency components at the downstream monitoring points include the blade passing frequencies of 15 fn and 30 fn. However, with the increase in radial deviation, the fluctuation amplitudes exhibit an asymmetric distribution, the uniformity of the pressure distribution in the circumferential direction of the labyrinth seal area becomes significantly worse, and the waveform of the downstream monitoring points changes significantly and presents a non-uniform distribution in one rotation cycle.

An analytical solution for convective heat transfer in conical gaps with either cone or disk rotating

This study is devoted to solving two problems of laminar fluid flow in a conical gap with small conicity angles up to 4°: cone rotation with a fixed disk, and disk rotation with a fixed cone. A new improved asymptotic expansion method for energy equation was used to obtain an approximate analytical solution to the convective heat transfer equation. The characteristic Reynolds number ranged from 0.001 to 1.0, the Prandtl number took values 0.71, 1, 5, and 10, and the exponent n* in the power-law for the disk temperature was 0 (constant disk temperature) or 2 (strongly radially increasing disk temperature). A novel model for the asymptotic expansion of the temperature profile and a novel expansion parameter Sv = Re2Pr, which is a new dimensionless number proposed for the first time in the known scientific literature, was developed. For the first time, new approximate analytical solutions were obtained for temperature profiles and Nusselt numbers on the disk and cone for both problems that agree well with the self-similar solution, if the Re and Pr numbers do not exceed threshold values. These analytical solutions are advantageous in analysis of experimental data and further development of one-dimensional models for gases, water, and aqueous solutions (Pr = 0.71–10).

Transverse vortex-induced vibration of two elliptic cylinders in tandem: Effects of spacing

Renewable energy converters, such as bio-inspired fluttering foils, are gaining popularity due to their eco-friendly properties. However, the system with multiple objects has received scant attention. Here, we analyze how spacing influences the transverse (one-degree-of-freedom) vortex-induced vibration of two tandem identical elliptic cylinders at a constant Reynolds number by employing a wide range of reduced velocities (Ur∈[2,14]) and space ratios (L∗∈[2,6]). The incompressible Navier–Stokes equations are solved using the overset mesh method in the OpenFOAM® library. The findings indicate that the wake structure goes through eight distinct wake modes, as well as two gap flow patterns (reattachment and co-shedding). Vibrational responses, force parameters, and flow patterns determine three spacing configurations. At a small spacing (L∗=2), the upstream cylinder (UC) has the traditional lock-in (the frequency ratio fy/fn≃0.95–1.05) at the reduced velocity (Ur≃7), and the downstream cylinder (DC) has a narrow lock-in region around Ur≃9. However, the UC has a wide soft-lock-in (the synchronization region of fy/fn≃1.15) at high reduced velocities (Ur≃8–10). Here, the transverse vibrations of both cylinders, but especially the DC, reach relatively high amplitudes. At a moderate spacing (L∗=3), the UC bears a lock-in zone analogous to a single cylinder with the same mass ratio, while the DC shows a vast soft-lock-in zone (Ur≃8–14). At a large spacing (L∗=4, 5, and 6), the amplitude of the DC is often larger than that of a single cylinder when it is in the lock-in region. The DC exhibits a peak in amplitude at Ur = 7 and a wake-galloping region for Ur &amp;gt; 12.

A Biphasic Fluid-Structure Interaction Model of Backflow During Infusion Into Agarose Gel.

The efficacy of convection-enhanced delivery as a technique to treat disorders of the central nervous system is limited by backflow, in which the infused fluid flows backwards along surface of the catheter rather than towards the targeted area. In order to improve treatment protocols, finite element models of backflow have been developed to understand the underlying physics. Garcia et al. (Journal of Computational and Nonlinear Dynamics, 8:011017, 2013) presented a finite element model that accounted for the flow in the annular gap that develops between the tissue and the outer surface of the catheter by using a layer of biphasic elements with a formula for the axial hydraulic conductivity to represent annular Poiseuille flow. In this study, we present a generalization of that model using fluid-FSI and biphasic-FSI elements that are recently available in FEBio. We demonstrate that our model of a 0.98-mm-radius catheter is able to reproduce experimental backflow lengths and maximum fluid pressures for infusions into a brain tissue surrogate and that it agrees well with the previous model by Garcia et al. (2013). The model predicts that the backflow length and the total amount of flow into the hemispherical region forward of the catheter tip is comparable for two different catheter sizes, albeit at a higher fluid pressure for the smaller catheter. This biphasic-FSI model has the potential to be extended to a stepped catheter geometry, which has been shown in experiments to be successful in controlling backflow.

Research on a Calculation Model of Cable Tension and Pumping Displacement of a Logging Tool String of Horizontal-Well Drillpipes

Summary With the vigorous development of unconventional oil and gas reservoirs, logging technology of horizontal-well drillpipes has become increasingly important. The improper selection of displacement during the pumping of drilling rods into horizontal wells can lead to accidents such as cable entanglement or tool-string pump detachment. Based on a comprehensive consideration of multiple factors such as drillpipe joints, small gap flow, pressure loss along the pipe, temperature, pressure, tool-string passability, and waterhole diameter variation, a cable tension analysis model for a logging tool string of a horizontal-well drillpipe is established. This model can predict a reasonable pumping displacement by analyzing the tension in the entire wellbore section, thereby avoiding cable entanglement or even tool-string pump detachment accidents during downhole operation. To verify the effectiveness of the model, we analyzed the cable tension during logging operations in Wells M1 and M2 using on-site test data and compared them with the measured tension data. The determination coefficient (R2), mean absolute error (MAE), and root mean square error (RMSE) were used to evaluate the degree of fit and accuracy of the model. The experimental results showed that the model can accurately predict the pumping displacement range before logging operations, ensuring that the cable does not entangle and the tool string is not pumped out during the downhole operation, thus providing effective methods and theoretical guidance for on-site drilling rod-pumping logging operations.

Impacts of wind flow across street-side building gaps on traffic pollutant dispersion at pedestrian level with different block heights

Urban neighborhoods, which contains numerous buildings and gaps between the buildings, are usually simplified as a single air-impermeable block in numerical simulations of urban air environments. Such a simplification reduces the computational costs but underestimates the pollutant removal capacity of the street, because the high-speed wind flow through the building gaps causes a flushing effect on pollutant distribution in the street. However, the influences of the gap flow on traffic pollutant dispersion remain unclear. Therefore, a four-way street network surrounded by four neighborhoods is assumed as a representative urban area, and the characteristics of the gap flow and its correlation with building height (H/W = 0.5–2, W = 30 m) are investigated numerically. The results show that the flow pattern within the street depends on the relative intensity of the flow across the gaps and the parallel bulk flow along the street. For lower (H/W = 0.5) or higher (H/W = 2) building heights, the gap flow is strong enough to break through the barrier of the bulk flow, and then enters the street to dilute the pollutants, causing the concentrations in the leeward region of the street to be lower than those in the windward region. Opposite results are found for medium building heights (H/W = 0.75–1.5), where the flow in the street is dominated by the parallel flow itself. The results also indicate that there exists a critical height, H/W = 1.5, below which the air environment is good in both the streets and neighborhoods.

Numerical investigation of the convective heat transfer coefficient for a concentric annulus with rotating inner cylinder with application in the deep geothermal drilling process

We present a numerical analysis of the convective heat transfer coefficient for the turbulent Taylor–Couette–Poiseuille flow in a concentric annular gap. The study utilizes rotational speeds defined by the Taylor number (Ta) ranging from 2×104 to 1×108, each combined with the axial flow defined by Reynolds number Re=4485, resulting in eight distinct simulation cases in addition to one base case where no rotation is involved. The transient k−ω model available with ANSYS FLUENT for computational fluid dynamics is used for turbulence modelling. The hydrodynamic and thermal fields of the model are validated by comparison with experimental and numerical results from the literature, with an overall qualitatively good agreement. Moreover, the resultant velocity fields exhibit an improvement over the LES results from the literature in comparison with the experimental data. The influence of the flow on the thermal fields is investigated by the swirl parameter N, which compares the rotation speed of the inner cylinder and the mean axial speed. There is no noticeable effect on the thermal fields for the magnitudes of N<1.19. Coherent structures are present for N≥1.19, confirming a direct effect of the temperature and the velocity profiles. The convective heat transfer coefficients (CHTC) are examined by their non dimensional form, the Nusselt number Nu, across the inner rotating wall and the outer static wall. The rotation of the inner wall shows a positive effect on the CHTC for both inner and outer walls. The correlation between the Nu and Ta delivers the exponents of 0.334 and 0.266 for the inner and the outer wall, respectively.

Study of nanofluid flow and heat transfer in a stationary cone-disk system

Rheometric, viscosimetric, bio-medical, and several other pharmaceutical machineries utilize the structural advantages provided by the geometry of a stationary conical diffuser. The problem of the Buongiorno nanofluid flow in the conical gap of a stationary cone-disk system for isothermal boundaries is studied. The governing system, comprising the incompressibility condition, Navier–Stokes equation, energy conservation equation, and conservation of Nanoparticle Volume Fraction (NVF) equation, is analyzed. The Lie-group theory has been used to derive a self-similar model. Solutions of the self-similar equations were computed numerically, and the expressions for the Nusselt number and Sherwood number are obtained. The parametric investigation reveals that the heat and mass transfer rate subside significantly when pre-swirl is introduced to the flow. Furthermore, the nanofluid slip mechanisms enhance the effective temperature of the system.

In Situ Observation of Bubbles and the Effect of Ultrasonic Vibration on Bubble Behavior in EDM.

Accumulation and concentration of debris in deep hole electrical discharge machining (EDM) significantly hinder its machining efficiency and accuracy. It is believed that the movement of bubbles associated with the discharge gap flow field play a pivotal role in debris removal and influence the discharge conditions. Ultrasonic vibration (USV) of the electrode is thought to be an effective method for improving EDM-generated bubbles and debris exclusion. In this study, we first elucidated the behavior of bubbles during EDM of holes with varying aspect ratios. Subsequently, USV was introduced to EDM. The behavior of dielectric fluid flow under the influence of ultrasonic vibration was analyzed using computational fluid dynamics (CFD), which revealed time-varying changes in discharge gap flow pressure and velocity. The velocity of the dielectric flow field near the electrode's side face was found to reach a maximum of approximately 15.2 m/s, greatly facilitating debris removal. High-speed camera observations revealed that bubbles were dispersed within the side gap, with most of them adhering to the electrode's wall. Furthermore, the bubbles exhibited a tendency to continuously break up and coalesce near the hole's outlet before escaping in the USV-assisted EDM. These observed characteristics of bubble behavior under the influence of USV are expected to significantly enhance debris removal and promote efficient dielectric exchange.

Experimental analysis of oil flow and drag torque generation in disengaged wet clutches

Fundamental knowledge of the oil flow in a disengaged wet clutch is essential for optimizing the cooling performance and the drag losses. However, no fundamental information on the oil flow and drag torque generation is available for dip-lubricated wet clutches. Therefore, the oil flow and drag torque generation in the sub-millimeter gap of a dip-lubricated wet clutch was experimentally investigated for three practically relevant oil levels. To enable optical access to the gap, transparent components were used. Further, a high-speed camera was used to capture the oil flow in the gap and grooving. Independent of the set oil level, the gap is oil-filled at low differential speeds, resulting in a single-phase flow. The drag torque increases approximately linearly with increasing differential speed due to the fluid shearing. In certain regions of the waffle grooving, air bubbles form locally. The air bubbles preferably occur in the grooves oriented in the radial direction, while the grooves oriented in the peripheral direction are filled with oil. Above a certain differential speed, the oil is continuously displaced from the gap, starting from the inside, due to the increasing centrifugal force. Consequently, the drag torque increases in a degressive manner until a maximum value is finally reached. The ongoing displacement of oil from the gap eventually results in a decrease in the drag torque. A steady drag torque is generated only when the oil is almost entirely displaced from the gap. Since the oil displacement from the gap already commences at a low differential speed, the cooling performance is limited for dip-lubricated wet clutches. The continuous displacement of oil from the gap can be held up, among other things, by increasing the oil level.