Shedding Period Research Articles

High Frequency Characteristics of the Near Wake and Vortex Past a Triangular Cylinder

Abstract The flow past a triangular cylinder is one of the fundamental flows and widely utilized in flame stabilization and heat transfer. In this study, the near wake and vortex characteristics of the flow past an equilateral triangular cylinder are experimentally measured by a high frequency particle image velocimetry (PIV) system at 3 kHz. The triangular cylinder is installed in a wind tunnel with Reynolds numbers ranging from 10,700 to 17,700. The Reynolds-averaged and phase-averaged methods are utilized to analyze the flow field. Based on the flow fields, the length of the vortex formation region is about 1.5 times of the length of the equilateral triangle side. The residence time of a vortex in the vortex formation region is equal to a vortex shedding period. The stream wise velocity of the vortex core center downstream the vortex formation is about 0.8 times of the freestream velocity, which is slightly larger than the value about 0.7 for the flow past a circular cylinder at the same Reynolds number. The maximum tangential velocity at the periphery of the vortex core maybe occurs slightly in advance of the vortex reaching the boundary of the vortex formation region. The normalized lengths of the recirculation zone of the triangular cylinder keep nearly unchanged and are about 1.55 to 1.9 times of those of the circular cylinder at the same Reynolds number. The normalized normal wise instead of stream wise turbulence intensity has stronger effects on the distribution of the normalized turbulent kinetic energy.

Comparable outcomes but higher risks of prolonged viral RNA shedding duration and secondary infection in cancer survivors with COVID-19: A multi-center, matched retrospective cohort study*

Abstract Objective To identify the differences in clinical features and outcomes between cancer survivors and non-cancer patients with coronavirus disease 2019 (COVID-19). Methods In this multicenter, retrospective, and observational cohort study from February 10, 2020 to March 31, 2020 in Wuhan, China, all cancer survivors infected with COVID-19 were screened, and statistically matched with non-cancer patients with COVID-19 using propensity score matching. Demographic, clinical, treatment, and laboratory data were extracted from a standardized medical recording system and underwent review and assessment. Results Sixty-one cancer survivors and 183 matched non-cancer patients were screened from 2,828 COVID-19 infected patients admitted to 4 hospitals in Wuhan, China. The median ages of the cancer survivor cohort and non-cancer patient cohort were 64.0 (55.0-73.0) and 64.0 (54.0-73.5), respectively (P = 0.909). Cancer survivors reported a higher incidence of symptom onset than non-cancer patients. Fever (80.3% vs. 65.0%; P = 0.026) was the most prevalent symptom, followed by cough (65.6% vs. 37.7%; P < 0.001), myalgia, and fatigue (45.9% vs. 13.6%; P < 0.001). The risks of the development of severe events (adjusted hazard ratio [AHR] = 1.25; 95% confidence interval [CI]: 0.76-2.06; P = 0.378) and mortality (relative risk [RR] = 0.90, 95% CI: 0.79-1.04; P = 0.416) in the cancer survivor cohort were comparable to those of the matched non-cancer patient cohort. However, the cancer survivor cohort showed a higher incidence of secondary infection (52.5% vs. 30.1%; RR = 1.47, 95% CI: 1.11-1.95; P = 0.002) and a prolonged viral RNA shedding duration (32 days [IQR 26.0-46.0] vs.24.0 days [IQR 18.0-33.0]; AHR = 0.54; 95% CI: 0.38-0.80; P < 0.05). Conclusion Compared to non-cancer patients, cancer survivors with COVID-19 exhibited a higher incidence of secondary infection, a prolonged period of viral shedding, but comparable risks of the development of severe events and mortality. It is helpful for clinicians to take tailored measures to treat cancer survivors with COVID-19.

MHD simulation of sawtooth events in meridional plane driven by various solar wind velocities

The meridional global MHD modeling of magnetosphere sawtooth events is performed for three values of the solar wind velocity: 200 km/s, 400 km/s and 800 km/s. The tooth cycle period of the open magnetic field specific flux decreases from 6.4 to 2.0 ks, respectively, while the oscillations amplitude changes inessentially. The tail magnetic field topology evolve through all the standard phases and events. In the distant tail the flapping current sheet is disturbed by the Alfven waves. In the dipole magnetic field domain the night-side attached vortices splitting and their subsequent shedding occur forming the Karman like vortex street. The vortex shedding period falls from 200 s to 110 s with the increase of the solar wind velocity. In the day-side shocked layer the vertically elongated thin magnetic island with the two neutral points is located. The magnetic island accounts for the deflection of the solar plasma inflow around the magnetosphere.

Factors Associated with the Delayed Termination of Viral Shedding in COVID-19 Patients with Mild Severity in South Korea.

Background and objectives: We aimed to analyze factors associated with the period of viral shedding in patients with confirmed COVID-19 who experienced only mild symptoms. Materials and methods: We conducted a multicenter retrospective study from three community treatment centers (CTCs) of South Korea. All patients included were admitted to the three centers before 31 March 2020. We collected data about clinical characteristics and the result of real-time reverse transcription polymerase chain reaction (RT-PCR). Results: Viral shedding was terminated within 32 days and 36 days in 75% and 90% of patients, respectively (range: 8–49 days). The mean period of viral shedding was 23.8 ± 8.7 days. In the multivariate Cox proportional hazards regression analysis, the existence of underlying comorbidities lowered the probability of the termination of viral shedding (HR = 0.561, 95% CI 0.388–0.812). Female sex and presence of COVID-19-associated symptoms also lowered the probability, but the significance was marginal. Conclusions: The existence of underlying comorbidities was associated with delayed termination of viral shedding in COVID-19 patients with mild severity.

Characteristics and outcomes of coronavirus disease 2019 (COVID-19) patients with cancer: a single-center retrospective observational study in Tokyo, Japan.

BackgroundAlthough severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused an international outbreak of coronavirus disease 2019 (COVID-19), data on the clinical characteristics of COVID-19 patients with cancer are limited. This study aimed to evaluate the clinical characteristics and outcomes including mortality and viral shedding period in COVID-19 patients with cancer in Japan.MethodsWe retrospectively analyzed 32 patients with a history of cancer who were referred to our hospital between January 31, 2020 and May 25, 2020. We evaluated the association between clinical outcomes and potential prognostic factors using univariate analyses.ResultsThe median age was 74.5 (range 24–90) years and 22 patients (69%) were men. A total of 11 patients (34%) died. Our analyses demonstrated that the mortality was significantly associated with lymphocyte count, albumin, lactate dehydrogenase, serum ferritin, and C-reactive protein on admission. The median period between illness onset and the first effective negative SARS-CoV-2 PCR result was 22 days (interquartile range 18–25) in survivors. Of four patients with hematological malignancy who developed COVID-19 within the rest period of chemotherapy, three died and the other patient, who received bendamustine plus rituximab therapy, had the longest duration of viral shedding (56 days).ConclusionOur study suggested that the risk factors for mortality previously reported in general COVID-19 patients, including lymphocytopenia, were also effective in cancer patients. Patients who received cytotoxic chemotherapy recently or were treated with chemotherapy, which can lead to lymphocyte reduction, had poor prognosis and prolonged periods of viral shedding.Electronic supplementary materialThe online version of this article (10.1007/s10147-020-01837-0) contains supplementary material, which is available to authorized users.

Impact of age on duration of viral RNA shedding in patients with COVID-19.

Background: The aim of this study was to investigate the host factors of patients with COVID-19 that were associated with delayed viral RNA clearance in specimens obtained from the upper respiratory tract.Results: A median of a 32-day period of viral RNA shedding was observed, ranging from 4 days to 111 days. On multivariate analysis, elderly age was independently associated with prolonged viral shedding (OR = 1.02, 95% CI: 1.01–1.04, P = 0.003). An incremental increase in the duration of viral RNA shedding was observed with increasing age (P < 0.05). The median (quartile) duration of viral RNA shedding was 23 (22) days (≤ 40 years), 30 (18) days (41–50 years), 33 (21) days (51–60 years), 34 (17) days (61–70 years) and 34 (17) days (> 70 years).Conclusions: Viral RNA shedding can persist for as long as 111 days in the upper respiratory tract. Increasing age is associated with viral RNA persistence.Method: The demographic and virological data of patients with laboratory-confirmed COVID-19 were retrospectively analyzed. A multivariate logistic regression analysis was performed to identify significant risk factors associated with delayed viral RNA clearance. The duration of viral shedding was compared among age-stratified groups.

Are the steroids a blanket solution for COVID-19? A systematic review and meta-analysis.

Steroids may play a critical role in the current pandemic of coronavirus disease-2019 (COVID-19), given the dearth of specific therapeutic options. This review was conducted to evaluate the impact of glucocorticoid therapy in patients with COVID-19 based on the publications reported to date. A comprehensive screening was conducted using electronic databases up to August 19, 2020. The randomized controlled trials (RCTs) and cohort studies evaluating the effectiveness and safety of steroids in patients with COVID-19 are included for the meta-analyses. Our search retrieved twelve studies, including two RCTs and 10 cohort studies, with a total of 15,754 patients. In patients with COVID-19, the use of systemic glucocorticoid neither reduce mortality (odds ratio [OR] = 1.94, 95% confidence interval [CI]: 1.11-3.4, I2 = 96%), nor the duration of hospital stay (mean difference [MD] = 1.18 days, 95% CI: -1.28 to 3.64, I2 = 93%) and period of viral shedding (MD = 1.42 days, 95% CI: -0.52 to 3.37, I2 = 0%). Systemic steroid therapy may not be effective for reducing mortality, duration of hospitalization, and period of viral shedding. Studies are mostly heterogeneous. Further RCTs are required.

Sustained Responses of Neutralizing Antibodies Against Middle East Respiratory Syndrome Coronavirus (MERS-CoV) in Recovered Patients and Their Therapeutic Applicability.

BackgroundZoonotic coronaviruses have emerged as a global threat by causing fatal respiratory infections. Given the lack of specific antiviral therapies, application of human convalescent plasma retaining neutralizing activity could be a viable therapeutic option that can bridges this gap.MethodsWe traced antibody responses and memory B cells in peripheral blood collected from 70 recovered Middle East respiratory syndrome coronavirus (MERS-CoV) patients for 3 years after the 2015 outbreak in South Korea. We also used a mouse infection model to examine whether the neutralizing activity of collected sera could provide therapeutic benefit in vivo upon lethal MERS-CoV challenge.ResultsAnti-spike-specific IgG responses, including neutralizing activity and antibody-secreting memory B cells, persisted for up to 3 years, especially in MERS patients who suffered from severe pneumonia. Mean antibody titers gradually decreased annually by less than 2-fold. Levels of antibody responses were significantly correlated with fever duration, viral shedding periods, and maximum viral loads observed during infection periods. In a transgenic mice model challenged with lethal doses of MERS-CoV, a significant reduction in viral loads and enhanced survival was observed when therapeutically treated with human plasma retaining a high neutralizing titer (> 1/5000). However, this failed to reduce pulmonary pathogenesis, as revealed by pathological changes in lungs and initial weight loss.ConclusionsHigh titers of neutralizing activity are required for suppressive effect on the viral replication but may not be sufficient to reduce inflammatory lesions upon fatal infection. Therefore, immune sera with high neutralizing activity must be carefully selected for plasma therapy of zoonotic coronavirus infection.

Kinetics of Viral Clearance and Antibody Production Across Age Groups in Children with Severe Acute Respiratory Syndrome Coronavirus 2 Infection

ObjectivesTo improve understanding of transition from viral infection to viral clearance, and antibody response in pediatric patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.Study designThis retrospective analysis of children tested for SARS-CoV-2 by reverse transcription (RT) polymerase chain reaction (PCR) and immunoglobulin G antibody at a quaternary-care, free-standing pediatric hospital between March 13, 2020, and June 21, 2020, included 6369 patients who underwent PCR testing and 215 patients who underwent antibody testing. During the initial study period, testing focused primarily on symptomatic children; the later study period included asymptomatic patients who underwent testing as preadmission or preprocedural screening. We report the proportion of positive and negative tests, time to viral clearance, and time to seropositivity.ResultsThe rate of positivity varied over time due to viral circulation in the community and transition from targeted testing of symptomatic patients to more universal screening of hospitalized patients. Median duration of viral shedding (RT-PCR positivity) was 19.5 days and time from RT-PCR positivity to negativity was 25 days. Of note, patients aged 6 through 15 years demonstrated a longer time of RT-PCR positivity to negativity, compared with patients aged 16 through 22 years (median 32 vs 18 days, P = .015). Median time to seropositivity, by chemiluminescent testing, from RT-PCR positivity was 18 days, whereas median time to reach adequate levels of neutralizing antibodies (defined as comparable with 160 titer by plaque reduction neutralization testing) was 36 days.ConclusionsThe majority of patients demonstrated a prolonged period of viral shedding after infection with SARS CoV-2. It is unknown whether this correlates with persistent infectivity. Only 17 of 33 patients demonstrated adequate neutralizing antibodies during the time frame of specimen collection. It remains unknown whether immunoglobulin G antibody against spike structured proteins correlates with immunity, and how long antibodies and potential protection persist.

Hair Growth in Brown Bears and Its Application to Ecological Studies on Wild Bears

Hair biomarkers have been widely used to study wildlife physiology and ecology. To apply this technique to practical studies, detailed information on hair growth is needed for each study species. This study measured hair growth patterns, including growth rate, periods of growth and shedding, and hair bulb structural changes in the brown bear. Hair growth was examined monthly in captive bears, and gross observations of fur were performed in wild bears. Guard hairs grew from late April to early October at an approximate rate of 0.5 mm/day for captive bears, and from late May to late September at an approximate rate of 0.7 mm/day for wild bears. Underfur began growing in August and continued into winter. The growth periods for intermediate hairs began at different times and lasted 3–5 months, depending on individuals. Most shedding was complete by August, and all shedding was complete by the end of September. Hair bulbs were classified into three types: white spheres (WS-type), black hooks (BH-type), and white hooks (WH-type). This classification can be used to discriminate between previous year's hair (WS-type) and current year's hair (BH-type or WH-type). The current findings will enhance the understanding of brown bear ecology in the wild.

The Effect of Vortex Shedding in Film-Cooling Processes

Film cooling using coolant jets is effective for increasing the cooling effectiveness of turbines. In the present study, large eddy simulation was employed for predicting small-scale vortices in the film-cooling process with different blowing ratios (BRs). The counter-rotating vortex pair (CRVP) was compared with the hairpin vortex, and results indicated the CRVP was the cross section of hairpin vortex. The results of a spectral analysis indicated a high correlation between the shedding period of the CRVP and the film-cooling effectiveness, and the dominant frequencies of the temperature and vorticity spectra were both identical to the shedding frequency of the CRVP when BR is less than 0.75. Furthermore, a higher shedding frequency of CRVP will suck the hot cross-flow into the core region more frequently, which will lead to a worse film-cooling effectiveness.

Exposures in adult outpatients with COVID‐19 infection during early community transmission, Tennessee

Exposures in adult outpatients with COVID‐19 infection during early community transmission, Tennessee

Prospective Study Comparing Deep Throat Saliva With Other Respiratory Tract Specimens in the Diagnosis of Novel Coronavirus Disease 2019.

BackgroundSelf-collected specimens has been advocated to avoid infectious exposure to healthcare workers. Self-induced sputum in those with a productive cough, and saliva in those without a productive cough have been proposed, but sensitivity remains uncertain.MethodsWe performed a prospective study in two regional hospitals in Hong KongResultsWe prospectively examined 563 serial samples collected during the virus shedding periods of 50 patients: 150 deep-throat saliva (DTS), 309 pooled-nasopharyngeal (NP) and throat swabs, and 104 sputum. DTS had the lowest overall RT-PCR positive rate (68.7% vs. 89.4% [sputum] and 80.9% [pooled NP and throat swabs]), and the lowest viral RNA concentration (mean log copy/mL 3.54 vs. 5.03 [sputum] and 4.63 [pooled NP and throat swabs]). Analyses with respect to time from symptom onset and severity also revealed similar results. Virus yield of DTS correlated with that of sputum (Pearson correlation index [95% CI]: 0.76 [0.62 – 0.86]). We estimated the overall false-negative rate of DTS could be 31.3%, and increased 2.7 times among patients without sputum.ConclusionDTS produced the lowest viral RNA concentration and RT-PCR positive rate compared to conventional respiratory specimens in all phases of illness. Self-collect sputum should be the choice for patients with sputum.

Effects of oral care on prolonged viral shedding in coronavirus disease 2019 (COVID-19).

AimTo assess the effects of oral care on prolonged viral shedding in coronavirus disease 2019 (COVID‐19) patients.Methods and resultsWe evaluated the clinical course of eight COVID‐19 patients, including their duration of viral shedding, by PCR testing of nasopharyngeal swabs. The average time from the onset of symptoms until the virus was no longer detectable was 31.6 ± 11.8 days (mean ± SD; range 17‐53). Thus, it took 15.1 ± 14.7 (1‐40) days from the time of clinical recovery for the virus to become undetectable. In two patients who had mental retardation and psychiatric disorders, the viral shedding period continued for 44 days or 53 days. These two patients did not voluntarily brush their teeth. When they were instructed on the importance of oral care, including tooth brushing and gargling, their tests for the coronavirus became negative.ConclusionMost of the patients with COVID‐19 had a viral shedding period of 30 days or less. In cases of prolonged viral shedding (≥44 days), noninfectious viral nucleic acid may have accumulated in uncleaned oral cavities and continued to be detected. We propose that tooth brushing and gargling remove such viral nucleic acid and improve the accuracy of PCR testing.

Don’t Miss the Other Side of the Problem in COVID-19 Research

We urge for the attention to the other side of the problem in COVID-19 research and propose investigations on the cases such as why some persons were not infected in an atmosphere of high coronavirus infectivity as others did, why in a 19 hour journey of a train to Tibet the coronavirus was not able to spread and infect any people during its period of peak viral shedding and transmissibility and in such a narrow and closed space. Based on the facts of two examples, we suggest that the research from the other side of COVID-19 may not only help to uncover the different aspects of COVID-19 mystery, but also provide important information and valuable methods to stop or slow the tempestuous worldwide pandemic of COVID-19.

Clinical course and treatment efficacy of COVID-19 near Hubei Province, China: A multicentre, retrospective study.

Currently, COVID‐19 has been reported in nearly all countries globally. To date, little is known about the viral shedding duration, clinical course and treatment efficacy of COVID‐19 near Hubei Province, China. This multicentre, retrospective study was performed in 12 hospitals in Henan and Shaanxi Provinces from 20 January to 8 February 2020. Clinical outcomes were followed up until 26 March 2020. The viral shedding duration, full clinical course and treatment efficacy were analysed in different subgroups of patients. A total of 149 COVID‐19 patients were enrolled. The median age was 42 years, and 61.1% (91) were males. Of them, 133 (89.3%) had fever, 131 of 144 (91%) had pneumonia, 27 (18.1%) required intensive care unit (ICU) management, 3 (2%) were pregnant, and 3 (2%) died. Two premature newborns were negative for SARS‐CoV‐2. In total, the median SARS‐CoV‐2 shedding period and clinical course were 12 (IQR: 9–17; mean: 13.4, 95% CI: 12.5, 14.2) and 20 (IQR: 16–24; mean: 21.2, 95% CI: 20.1, 22.3) days, respectively, and ICU patients had longer median viral shedding periods (21 [17–24] versus 11 [9–15]) and clinical courses (30 [22–33] vs. 19 [15.8–22]) than non‐ICU patients (both p < .0001). SARS‐CoV‐2 clearances occurred at least 2 days before fatality in 3 non‐survivors. Current treatment with any anti‐viral agent or combination did not present the benefit of shortening viral shedding period and clinical course (all p > .05) in real‐life settings. In conclusion, the viral shedding duration and clinical course in Henan and Shaanxi Provinces were shorter than those in Hubei Province, and current anti‐viral therapies were ineffective for shortening viral shedding duration and clinical course in real‐world settings. These findings expand our knowledge of the SARS‐CoV‐2 infection and may be helpful for management of the epidemic outbreak of COVID‐19 worldwide. Further studies concerning effective anti‐viral agents and vaccines are urgently needed.

Pathogenicity and transmissibility of clade 2.3.4.4 highly pathogenic avian influenza virus subtype H5N6 in pigeons

Pigeons were previously thought to be resistant to H5 viruses and to play a minimal role in spreading these viruses. In this study, we evaluated the pathogenicity of two clade 2.3.4.4 H5N6 viruses in pigeons and the potential viral transmissibility to specific-pathogen-free chickens in direct close contact with experimentally infected pigeons. No pigeons from the A/goose/Eastern China/Xin/2015 (GS/Xin) group exhibited clinical signs or mortality, and the virus was only detected in a few organs. However, 3 of 12 pigeons inoculated with the A/goose/Eastern China/0326/2015 (GS/0326) virus died, and 7 of 12 showed neurological symptoms and efficient viral replication in multiple organs. In both groups, viral shedding occurred in only some of the pigeons, the shedding period was relatively short, and the infection was not transmitted to the chickens. We also used chicken, duck, and BALB/c mouse models to evaluate the pathogenicity of the two H5N6 isolates. Both H5N6 isolates showed highly pathogenic to chickens but different degrees of pathogenicity in mice. Interestingly, in ducks, the intravenous pathogenicity index indicated that the GS/Xin isolate was low pathogenic, and the GS/0326 isolate was highly pathogenic, corresponding to the pathogenicity in pigeons. Our results indicated that the pathogenicity of the clade 2.3.4.4 H5N6 virus is diverse in pigeons, and pigeons contribute little to its transmission among poultry. However, pigeons may still be potential healthy reservoirs of the H5N6 highly pathogenic avian influenza virus.

Protection of layers and breeders against homologous or heterologous HPAIv by vaccines from Korean national antigen bank

Korean government has selected and stocked five type antigens of two clades as Korean national antigen bank having high possibility of introduction to Korea. We aimed to evaluate the efficacy of the clade 2.3.2.1c and 2.3.4.4c H5Nx vaccines from the Korean avian influenza (AI) national antigen bank for emergency preparedness for their potency and protective efficacy against lethal homologous and heterologous viruses in layer and breeder chickens practically. The PD50 (dose of vaccine that protects 50% of chickens from viral challenge) of all vaccinated groups was >50, which was satisfied with minimum antigen requirement of OIE, and the PD50 levels of the two vaccines differed depending on strain and chicken breed. In homologous challenge, all vaccinated groups exhibited 100% survival with no clinical symptoms and high levels of pre-challenge protective immunity (7.2–8.5 log2), although they did not completely prevent virus shedding. On the other hand, against heterologous virus challenge, vaccinated animals exhibited 62.5–80% survival with lower antibody titers (2.3–3.4 log2) and a longer period of virus shedding (14 days post infection [dpi]). Our results suggest that the clade 2.3.2.1c and 2.3.4.4c H5Nx vaccines are good candidates for emergency vaccination of commercial chickens and support the idea that close genetic matching between vaccine and challenge virus provides the best protection.

Symptomatic Infection is Associated with Prolonged Duration of Viral Shedding in Mild Coronavirus Disease 2019: A Retrospective Study of 110 Children in Wuhan.

Information regarding viral shedding in children with coronavirus disease 2019 (COVID-19) was limited. This study aims to investigate the clinical and laboratory characteristics associated with viral shedding in children with mild COVID-19. The clinical and laboratory information of 110 children with COVID-19 at Wuhan Children's Hospital, Wuhan, China, from January 30 to March 10, 2020, were analyzed retrospectively. The median age was 6 years old. The median period of viral shedding of COVID-19 was 15 days (interquartile range [IQR], 11-20 days) as measured from illness onset to discharge. This period was shorter in asymptomatic patients (26.4%) compared with symptomatic patients (73.6%) (11 days vs. 17 days). Multivariable regression analysis showed increased odds of symptomatic infection was associated with age <6 years (odds ratio [OR] 8.94, 95% confidence interval [CI]: 2.55-31.35; P = 0.001), hypersensitive C-reactive protein >3.0 mg/L (OR 4.89; 95% CI: 1.10-21.75; P = 0.037) and presenting pneumonia in chest radiologic findings (OR 8.45; 95% CI: 2.69-26.61; P < 0.001). Kaplan-Meier analysis displayed symptomatic infection (P < 0.001), fever (P = 0.006), pneumonia (P = 0.003) and lymphocyte counts <2.0 × 10/L (P = 0.008) in children with COVID-19 were associated with prolonged duration of viral shedding in children with COVID-19. Prolonged duration of viral shedding in children with COVID-19 was associated with symptomatic infection, fever, pneumonia and lymphocyte count less than 2.0 × 10/L. Monitoring of symptoms could help to know the viral shedding in children with COVID-19.

Experimental study of the evolution of water-entry cavity bubbles behind a hydrophobic sphere

This paper describes an experimental investigation of the cavity evolution and shedding wake behind a hydrophobic sphere during the water-entry process. Two distinct shedding phenomena are confirmed by varying the impact velocity and sphere size: regular air-bubble shedding and unstable air-cloud shedding. Both of these modes are highly dependent on the Weber and Bond numbers. Under the air-bubble shedding mode, approximately periodic big bubble shedding and low-frequency oscillation signals are observed. The relationship between big bubble shedding events and the corresponding acoustic signals is derived, and an empirical method for predicting the shedding period is proposed. The in-phase relationship between small bubble shedding and cavity rippling is confirmed, and we refer to the cavity shedding phenomenon as “acoustic” shedding. Unlike the observations of air-bubble shedding, the air-cloud shedding mode produces a group of disordered small bubbles from the rear of the cavity. Moreover, the cavity seal type has a significant effect on the cavity shedding mode. A deep seal always promotes the onset of air-cloud shedding, whereas surface seals with relatively low Bond numbers result in the air-bubble shedding mode. A surface seal suppresses resonance in the cavity volume. By observing the cavity motion, we find that air-cloud shedding is always accompanied by severe cavity resonance and a rapid decrease in cavity length. Under the air-bubble shedding mode, the cavity motion exhibits relatively weak oscillations.